Why does my dog Alert to OTHER Detection Dog odors??

We get this question often. Handlers run joint training with other detection dogs from other disciplines and low and behold their dogs also alert to those other hides... Why is this? Do you need to now proof off these other odors?
Some of you also deal with dogs alerting to a distractor or proofing odor you placed in the area. I also talk about why this may happen and a better way to see if you really need to proof your dogs off a particular odor or not.
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