Was the dog left at the neighbor’s house or at the sitter’s house, with the neighbor watching? If second, I don’t see a problem unless you (1). specifically told the sitter your pup couldn’t be introduced to anyone else during their stay or (2). It was discussed that the sitter couldn’t leave for 2 hours (unlikely).

Unless the dog has special needs that are specifically outlined, it should be assumed that life at the sitter’s house will carry on as normal during boarding and doggy daycares. As in, they will run errands, go to the gym, roommates or friends might come and go. Dogs with anxiety often benefit from being socialized (proper protocol/expectations should always be discussed beforehand though). Unless the sitter was leaving your dog with someone else for a period longer than you had discussed (I personally try to not leave for longer than 2 hours during daycare), or you explicitly told sitter to keep dog away from others, I think it’s okay. Especially if nothing happened? Just communicate clear expectations next time that even if someone else may be nearby, you’d prefer the pup be left alone while the sitter runs their errands.