Two families in our neighborhood have gone back east for the holidays. As a result, we are cat-sitting a pair of kitties across the street and also keeping an eye on the very friendly indoor-outdoor cat next door (its magnetic collar allows it to use our cat door as well as its own).
Friday night I went over to feed and check on the pair across the street and was greeted with loud meowing as one of the cats attempted to slither past me out the front door. I managed to get the door closed without letting the cat out, vaguely aware of loud meowing behind me in the kitchen. I turned around to see the two cats standing in the kitchen, meowing. But there was still meowing coming from an unexplained third cat, standing at my feet.
Somehow, the extra cat (the one belonging to our next door neighbors) had gotten into the house -- but couldn't get out again. It looks nearly identical to one of the pair that lives in the house, so my guess is that someone (the cleaning person?) had stopped by and, thinking it belonged there, let it in.
I let it out, and everyone was much happier.