Sourcing, Residual Odor and Preventing False Alerts FAQ – Part 1

Question: Carla, can you suggest training exercises to help work through dogs (false) alerting on trace or residual odour? I assume that for the most part it’s team inexperience but now that the sport of nosework is getting so popular facilities have multiple people placing hides and I’d like some pearls of your wisdom/experience to help my dogs and myself not to call alerts not at source. Thanks!

Answer: You can train your dog not to alert on residual odor. But don’t focus only on what you DON’T want. Also train what you DO want: precise sourcing. This video shows some tests that will help you assess your dog’s ability to go to source so you can create a plan. 

Before we get to the tests in the video, we need to define some of the terms first. If you want to help your dog, it’s critically important to understand these concepts! In recent nosework classes, when I asked students what sourcing was and only two answered correctly. As all the teams warmed up on boxes, many rehearsed common errors:

  1. Didn’t work to source, even when I pointed out the hot box in advance, and it was clearly labelled so they could read it
  2. Didn’t mark when their dog got to source
  3. Didn’t reward the dog at source, but a foot away, and
  4. Didn’t cue the dog to leave source.

Whenever you practice, you are building a behavioral chain. Once learned, it’s hardwired into the neurons of the dog’s brain and is difficult to retrain.

Don’t Build Flaws Into Your Training

Every search is an opportunity to practice sourcing (or rehearse undesirable behavior). How you define success and choose what to reward is crucial. You can prevent problems by working to source from the dog’s very first searches so he clearly understands his job is to get his nose as physically close as possible to source.

Are you happy with close? You get what you reward. If you reward your dog for being close, that is what you’ll get. If you don’t know where source is and you don’t reward for finding it, you are setting your team up for difficulties in future.

One problem is that students want to help their dogs be right. They reward too soon, or too late, confusing the dog with inconsistency. At your first novice level competition, when boxes are 5 feet apart, and all you need to do is to identify which box is hot, it is fairly easy to pass just by noticing which box the dog is most interested in.

Then you move up. Criteria get harder in order to pass at the higher levels of nosework competition. The objects can be touching each other, adjacent to distractions, and the judge may ask where source is if you call an alert. Knowing that it’s “on a vehicle” is insufficient to pass. You’ll need to pinpoint the location of source e.g. down to a corner of the license plate. Fringe alerts (not close enough to source) or false alerts on residual odor frequently result in failure.


What is Residual Odor?

Residual odor is essentially scent remaining in the air after the hide is moved to a new location.

When you move a hide, you don’t remove all the odor. In Police K-9 Magazine, Ted Daus explains: “The term residual means: the quantity remaining after most of something has been removed. Odor is a distinctive smell. Thus, “residual odor” is what remains of a distinctive smell after the original object has been removed” (1). He goes on to comment that: “Dogs alert to odor and direct the handler to the source of the odor, and then the officer searches for drugs… Every handler has had the experience of his dog alerting to a vehicle in which odor was present but no drugs were found. That should not shock anyone when it happens, nor should it shock the judge or the lawyers involved in the case once they learn that a dog does not find drugs but merely alerts to drug odor”.

From the detection dog’s perspective, if cocaine was in a vehicle for a prolonged period, and removed right before a search, the scent persists after source has moved. Nobody vacuumed up all the scent in the air! He’s not entirely wrong because odor is present. That’s how every successful search starts: encountering the odor plume and then searching for source. But problems can arise if the dog indicates the presence of source when the drugs can’t be found. In a messy world, you won’t always know when residual odor is present. That’s where double-blind testing with known hides comes in for testing the dog’s abilities and achieving certification.

Another example that may be confusing is when a hunting dog detects where a bird laid down on the ground, perhaps even leaving feathers and scat, after the bird has flown away. In training, gundogs aren’t rewarded for finding that trace and/or residual odor, but rather, for only for finding source: the live bird.

No matter the target odor, part of training a reliable detection dog is training him what not to find. For example, bed bug detection dogs need to learn not to indicate where bed bugs have been, ignoring even bed bug spotting/feces, bedding, shed skins and dead bugs (situations were extermination is not required). They are only rewarded for finding live bugs (where treatment is required).

What is Sourcing?

Sourcing is essentially finding the precise location of the highest concentration of target odor: the live bugs or bag of drugs. The best scent dogs understand that they should work to source or as close as physically possible. It makes the handler’s job of finding the evidence easier.

We train dogs to source in a scent detection lab. The lab presents many boxes with holes, and the hide is taped securely inside the box, right next to the hole. That way the handler and dog both agree that the location of source is precisely at the hole. This visually distinct and crystal-clear criteria enables handlers to work to source, mark the instant the dog gets to source, and deliver repeated rewards at source.

Only the precise location of the hole is correct. The rest of the box is incorrect. And holes of adjacent boxes, only 1 inch away, are also incorrect.

A dog that doesn’t understand he needs to go to source may detect the odor plume and perform behaviors that the handler recognizes as an alert, when the dog is only close (but not at source). That’s rehearsing odor detection but not following the plume to get to source. When teams continue to rehearse those behaviors by rewarding away from source, they may end up seeing problems like vehicle searches where the dog runs to the vehicle and lies down looking at the handler, who can’t determine where source is.

How to Test Your Sourcing

So how proficient is your dog at sourcing today? Don’t wait until double-blind test or competition to assess your dog’s skills. If you test and fail more at home, then train through any issues, you’ll build trust in your team and be more prepared for the challenges you’ll face in blind searches later. Try the simple tests below to get an accurate assessment of your current capabilities.

Don’t forget, to train or test sourcing, you need to know the exact location of source. Don’t worry about the visibility of the hide yet. Secure your hide so it does not move! You don’t want to reward the dog when you think he’s at source, only to find that the hide fell and is actually somewhere else. If you make that mistake, you’re practicing false alerts on residual odor.

You can only work on 1 thing at a time. Mark and reward the sourcing. Indications, distractions, and other challenges should be covered in separate sessions.

Sourcing Test #1 – One Hot Box

This should be your first search of the day. Do it cold if you want a challenge, or warmup to increase your chances of success. Set up in a distraction free environment. Place 1 hot box with a hole right next to the hide. Give your dog the search cue and wait at a distance. Do not assist your dog! If you’re always helping, you’ll never get a true test of your dog’s ability.

The ideal performance is when the dog searches from the instant you give the cue, moves confidently and quickly to put his nose in the hole. If your performance is something else, decide if there’s a hole in your training that warrants attention.

Sourcing Test #2 – Tower of Boxes

Repeat the test on a tower of boxes with holes, where the hot box is at nose level. Are you happy with your dog’s performance?

Problems may include:

  • Slow, uncertain, distracted dog that sniffs distractions or repeatedly looks at the handler for direction
  • Multiple search cues
  • Passing the hide repeatedly
  • Spending a lot of wasted energy getting to source, or
  • Dog that runs over to the boxes and lies down to indicate without going to source

Sourcing Test #3 – Move the Hot Box to Create Tempting Residual Odor

Right after test #2, move the hot box up to the top of the tower of boxes. (Generally, nose level hides are easiest while high hides are more difficult.) Do not age the hide, but start your search right away. Your dog will be tempted to false alert on the residual odor remaining in the air lower down. Are you happy with his performance or does your dog fail to work to source?

Dogs who underperform in these tests may benefit from practicing sourcing. In part 2, we’ll show you how.

How did you do when you tested your dog’s sourcing? Drop us a line in the comments below.


  1. Ted Daus (2013)”When does drug dog odor become “residual”?”. Police K-9 Magazine [web page]. Retreived from:
Media by Hunter’s Heart, Jill Gibbs. 
About Dr. Carla Simon (132 Articles)
From bed bugs to birds, from narcotics to nosework, Dr. Carla Simon, MD, BSc, MBA has trained thousands of students around the world, since 1999.

2 Comments on Sourcing, Residual Odor and Preventing False Alerts FAQ – Part 1

  1. Jim Bryson // May 8, 2018 at 7:09 pm // Reply

    Thank you Carla! This just what we needed in our training right now


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  1. Sourcing, Residual Odor and Preventing False Alerts FAQ – Part 2, Hunter's Heart, Canada

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