[envisio]

Many of these dogs will "alert" when they know the handler wants them to alert

Or the officer will say they alerted when the dog did not.
I saw an officer during security training that said the dog alerted. The suspect said "No he did not. I watched him. He did not do any such thing". The suspect was right, the dog did not alert but the cop said "oh yeah, he alerted, we can search now."

[T-Bone Texan]

Cops will also, at a stop, rub their finger on a car as they initially approach it.

We know now that this is to create false positives.

What is on substance on the cops’ finger? BBQ sauce? Bacon grease?

I don’t know, but when you view the videos it is apparent that dogs love it.