Why?

I’m going throw divorce at the moment. I’m still staying in the flat and he pays for the mortgage and everything. Now there is three keys for the house. I’ve one.. he have one.. and there is one we usually keep at his parents house. I’ve lost my key a while ago and he gave me the key his parents have.( before we separate ) Why is he now insisting on finding the other key! Is there anything he can do regarding selling the flat? It just doesn’t feel right him asking to have the other key that much!