Very interesting! Smoke is sometimes used to show how odor might move within a nosework environment. But in this video, they put a smoke source on a Barn Hunt course!
The smoke here is likely more intense than rat odor, but it gives you an idea of how the rat odor might fill a course and the dog needs to locate the source (strongest point) to locate the tube. It also shows how very slight breezes (real or from HVAC) can affect the odor (as well as other conditions that affect air flow). The dogs that sniff from the start box ARE likely getting rat scent, it just takes a lot of experience for them to be able to follow the faint odor there straight to a source/hide. And it takes dogs some practice to be able to sort out the lingering odor from a current hide. What I’m not sure of is how they contained the burning smoke bomb in a way that doesn’t risk setting the straw on fire! https://www.youtube.com/watch?v=pfcteVTDmNw&list=TLGCLsaxJmJ0PYExNjEwMjAxNg