I don't know why it bothers me so much, other that I almost feel like it's an invasion of my privacy even though most of his stuff is here and we do own the house together.
I feel like this ALL THE TIME. I have tried asking H to let me know when he is coming over. On some level they still think of the house as theirs. I have used reason (you don't contribute to the mortgage, I would not just show myself into your flat), legal position (we live under different roofs and legally you have to ask permission or be invited), but is like talking to a brick wall. He complies for a while, but then one of the children will forget something for school, and he will just show himself in to get it and then we start the cycle again. He finds reasons to come back. The dog needs to be walked, the grass needs to be mowed, the builder needed to talk to me, I needed to drop something off blah blah blah.
H was watching D8 a lot last week as her school were still on holidays. We agreed that he could watch her at home because this is where her toys etc are. Whilst he was here he 1. cleaned out the loft 2. painted a ceiling 3. cleaned my car (again) and 4. sorted through D12's clothes.
All very nice of him to do. But weird. I thank him (one of my 180's is to be more appreciative) and then say "can you let me know when you are coming over". Sometimes he listens. Sometimes he doesn't.
Say something like "Thank you for mowing the law. In future can you let me know when you are coming over'. The first part clearly sets out that the house (and the lawn) is yours and by cutting it he was doing you a favour. The second part states very clearly what you want.