I went with a group of people to an event a while back. Actually, I went early to help set up. We were renting space from another group that had a large room with several smaller rooms to the side. I was one of the first persons in the building and noticed a house cat sitting in the hallway. It was smokey gray and was watching the door as we came in. When it saw us, it stood up and walked towards the smaller rooms then turned and headed down the hallway away from us; I got the impression that we were not what it expected to see. As it got to the last door, I realized that I could see through the cat just as it turned and trotted through the door. This happened just as I was about to alert my group that there was a cat inside the locked and otherwise empty building.
In the past week I've noticed a lot of activity close to ground level in my apartment. About the right size, shape, and speed of a house cat. There is no cat in the apartment building.