I live in an apartment at the 12th story of a high-rise residential apartment building in a typical Chinese city. The building has elevators and stairwells. In every story of the building, there are tens of apartments along the long hallway. About 2 months ago, I encountered a cat when I was about to go out from home and waiting for the elevator. When the elevator arrived and door opened, this cat just popped out of the elevator. He followed me and just sit at my doorstep, seemingly wanting to go inside, so I let him. I then adopted this cat whom I think was abandoned by others (which is not uncommon) in the same residential area.
First Time Letting Go
As time goes by, he became increasingly eager to going out. Several days ago, I let him go. He knows where the stairwells are and where it leads to (i.e., "the outside world"). (The experience was gained by his former "failed" attempts to "escape".) So he went by himself, like the same voluntary way he came to me at the beginning.
Another day I saw him outside and "miao"-ed at him. Then he just came back with me. (He went in and out elevator swiftly with me, and sit at my doorstep waiting for me to open the door.)
The Second Time Letting Go
He came back, ate, drank, and sleep. After less than 48 hours, he seems to want to go out again. As before, I let him go.
The Unexpected and The Question
Yesterday afternoon, less than 2 days after his second leaving, my girlfriend opened the door as we were about to go out. The cat was just lying there waiting for us to open the door so that he could go in.
We are really shocked at this. Therefore I wish to know:
Is it normal for a cat to manage to go back "home" in a situation where s/he has to climb an exact number of stories to get the correct floor and find the correct door among tens of other doors?
How does he know which story to go for and which door to stop at? Is it by scent?
Any information is appreciated. Thanks in advance.