I kind of think that also may be a factor in false alerts, however I did have an experience that demonstrated just how good these dogs sense of smell is. I once gave a ride to a Young lady who was having heroin withdrawals. She threw up in my van and apologized like crazy. I really felt bad for her but we did get her off that crap. Anyway, over a year later I got pulled over in another state and the officer decided he needed to get a police dog in my van. Police dog found nothing as I expected until it got to that spot on the carpet and alerted by pawing the spot and then stood over it waiting. Cops pulled the carpet up and found nothing except a slight stain. Then I remembered that sick Young lady. Gee, that had to be a very faint trace.