You all seem to be assuming the use of them is prompted by a specific concern, rather than just the notion that you don't know this person at all.

If I suspected abuse etc I wouldn't leave my child with them either.
True. I know when I was babysitting for strangers, I was asked to cone early and sit and play with the child/ren and chat with the parents. Some people didn't seem to care at all and I'd just get there and they'd leave. Hmm, maybe they WERE filming me?