Why dont they take their belongings?

I thought it was just me!
My stbx has taken a lot of his things and seems to be using the marital house as a storage facility. As we are sorting the finances out, he still has a key to the house. I have asked him to be specific about what he wants or to take the whole lot - I'm not precious about our jointly purchased stuff. My theory is that he is still marking his territory when he comes in and just helps himself to stuff now and again. Its to undermine and unsettle us so that we are aware that it is still partly his. Its just little things sometimes like taking photo albums, dvds, books, the iron, salad spinner. without asking - not a lot, but enough to make you feel a bit insecure as things come and go. He also likes to come in and charge his phone and use Netflix, and p*ss on the toilet seat (every time!)

But I think they also leave things because they like to think that everything is going to stay the same.