So Inappropriate
It’s not unusual to trust a neighbor with a spare key for emergencies especially when you’re away from home. This incident, however, is plain creepy because the OP was absolutely clueless. Apparently, their neighbor decided to use their “hide-a-key” to borrow some milk for their coffee and only told the OP about it AFTER they were done.
We know what you’re thinking – that’s practically a break-in and it is! The worst part was that they didn’t even seem sorry about it. In fact, they didn’t think there was anything wrong with what they had done. We can only hope that they changed the locks after this unpleasant episode.