Cat behind plant
I went down to the third floor, preparing to leave and thinking that I had found the black and white cat in question that everyone mentioned, when I saw THIS black and white cat. It was sitting on a shelf next to these potted plants along with a ginger and white cat. The other cat scooted off and ran into a flat through the open window, but this cat did not.
No one was home, so I spoke with a neighbour and SHE had cats as well, though she had plastic on the bottom of the gate and she said her cats never went out (I didn't see any cats while I was speaking with her in her flat). She said these two cats were her neighbours and she would speak with her and pass her my message.