Can I Change The Lock On An Exterior Cellar Door If The Previous Owner’s Son Says His Family Still Has “things Down There”?

Location: Missouri

I bought an older house last fall. It has one of those partial basements with an exterior storm cellar style door on the side of the house, plus normal basement access from inside. During inspection the cellar area was mostly empty except for some shelves, old paint cans, and junk the seller agreed to clear out before closing. After I moved in, I started noticing that the exterior cellar door was sometimes not fully latched even though I barely ever use it. I assumed the wood had swollen or I had shut it badly. Then about three weeks ago I came home from work and found a man coming out of that side gate carrying a plastic tote. He scared the hell out of me. He said he was the previous owner’s son and had "a few family things" stored in the cellar that his mother forgot about. I told him he could not just let himself into my yard. He acted offended and said they had always used that entrance and he still had an old key. I told him this is my house now and he needs to stop. He left, but not before saying there were personal documents and some of his late father’s belongings down there.

The part making me hesitate is that when I checked the cellar after that, there actually were a few bins tucked behind the stairs that I had not noticed before, plus an old footlocker and a dusty framed photo. Nothing valuable looking, but clearly not mine. I texted my realtor, who contacted the seller’s agent, and the answer that came back was basically "they thought everything had been removed." Since then I have seen the son’s truck slowly pass my house twice, and yesterday the cellar door was cracked open again by maybe an inch. I do not know if that means he got in again or if I am getting paranoid, but I have reached the point where I want to change that lock immediately and put his stuff in the garage until someone formally arranges pickup. My wife thinks we need to call police first because if he still has a key maybe this becomes more complicated than plain trespassing. Am I allowed to secure that entrance right now and give written notice for them to collect whatever is left, or do I have to store it a certain way or wait some specific amount of t ime before treating it as abandoned property?