We had an agency sitter with all the quals and checks and she still did cause problems for dd1. It's just made us very cautious, most likely we won't use someone we dont know again. The girls go to daycare so I don't have a problem with that, but it feels different somehow.
Would you consider asking any of the daycare people to waych your DD's if the need arose.
There are a few young ones at ours who are always after the extra cash, and our children adore them.

Would I use a nanny cam? I am not sure. I suppose if the need arose that I had to use a complete stranger through and agency, then for peace of mind yes (and it would have to be linked that I could check it whilst out, not after the event). We don't have any family near by (well my Dad is 2 hrs away but talking of moving on), so if I was to get sick, we would need to hire someone if it was too long to rely on friends and I would want to know they are safe and getting along with my children.