I've been to the post office twice this week to check a po box, and both times I didn't notice any security cameras in the lobby. I'm surprised there aren't any, and it'd be perfectly normal and natural to have them installed without people thinking anything suspicious. Most people would just assume they are there for security. To me that seems a lot better and less likely to arouse suspicion than some secret camera nonsense.
I agree, cameras are so commonplace that if they wanted to monitor people inside the post office, no one would blink an eye at a big fat camera pointing at the counter.
