A very cat keeps visiting my neutered cat. She burbles and waves her bum at him - and yowls at night outside the window. This has been going on for a few days. She's not hungry and she wears a collar. I've emailed everyone I know in the building but nobody knows her. I'm pretty sure somebody in a nearby flat owns her and hasn't bothered to get her neutered. The signs of her being in heat are pretty obvious. Any suggestions?