This is another experience from our place back in Tromsø, Norway, not only on one occation but on several over the years we would continuously return home to find our cat inside our locked apartment after we took him out with us and left for work, store, university or where ever.
At first we thought our landlord, that lived upstairs from us might have been nice enough to lock our cat in for us while we were away, but when we asked he said that we had the only keys, and even if he had keys he would let the cat stay outdoors until we get home.
You might think like we did that he might enter some other way, like through an open window or a ventilation shaft, we though about that too but on none of the occasions was any of the windows open, they were all securely locked, and our cat was not able to get in through the ventilation shafts since all of them were blocked so they would let air in but keep any animal out (like a rat, cat or bird) secondly all the ventilators to the apartment was too high up for our docile and heavy set cat to climb up to and to narrow for him anyway. He never even climb trees.
The most compelling happening was when our landlord returned back from vacation after two weeks and unlocked his own apartment upstairs to find our cat inside waiting for him, he was totally shocked to find the cat there and could see that he was in good shape and would not been what one would expect if he been there for two weeks locked up, so when he brought our cat down to us, we could inform that we had just put him outside a while earlier so it seemed totally impossible for him to just appeared into his apartment like that, but still that was exactly what we had experienced so many times our self.
Some would say he could have sneak in without you noticing him, but I know for sure we would have noticed him, and since we were aware that might been the case we made sure he was not around when we opened the door to remove any doubts in case he made another surprise entering, which he continued to do.