Random sims walking through house/apartment

This just happened to me. Apartment in San Myshuno. My Sim has his front door locked to all. He was in the kitchen when suddenly a random NPC appeared in his bedroom, materializing through the wall, running across the bedroom, then disappearing through the opposite wall.

Then a few minutes later another Sim suddenly appeared out of nowhere in the middle of his bedroom. She spent an hour (of sim time) walking around in a circle holding a glass. My Sim had never met her before.