When my first son was small I was always nervous leaving him with the various available teenagers who offered to baby sit and would phone quite often if away for the evening. It did not help that he would howl the whole time I was going out the door and I would hope that he did not have bad memories of previous baby sitting occasions.
I may have considered using nanny-cameras to check up to see if the babysitter was neglecting him or having her boyfriend over, or perhaps watching television and not attending to the child. I am not sure if I would have advised her that I was using nanny-cameras, although I think the law in Canada would dictate that I would have to do so. I know that I would not like to be recorded without my knowledge, we all have little habits that are not for public consumption, although not harmful in themselves.
If I had seen anything even slightly shady I would have been speeding back home to kick her out of there and be glad I had used nanny-cameras to keep my child safe.