Board Thread:False Info and Speculative Discussion/@comment-25176701-20150818031151/@comment-13965604-20150821135440

Hm. Ok. But that still doesn't mean that they know what it is. Stiles is the only one who was around when Peter said it, he may not have told any one. And Derek didn't exactly seem to get down to details like that.

I can't recall if they ever described the scent up on the roof, but we know Stiles smells like it all the time, and extra bad a few episodes before Donovan, I imagine Stress would be acrid too, though it isn't canonically stated. It can't be a pleasant smell though. So if they don't know what to look for or think they need to look for it, they may not pick it up from smelling Stiles.