Neighbor's keys, what would you do?

I have a newer neighbor, moved in maybe 4-6 months ago. This person is younger and not the best neighbor, but that's a different topic. He lives across the hall from me. I have seen several times where he has left his keys in his door, which I'll outline. Please keep in mind that my area is a Hotspot for covid and we are uncomfortable coming close to or interacting with those outside of our home unit as of now.

The first time: It was late at night, I knocked on his door to try and return them but no one answered, so I took them and returned them in the morning.

Second time: It was again late at night again, the keys were placed under his door mat and a note left.

Third time: At this point, neighbor isn't learning to take his keys in so we decide it's not our problem, he needs to protect his own home and self. We left the keys dangling in the door and watched to see how long it took for him to notice. He went 3 days before removing the keys.

Fourth time: Today I left and noticed them in the door. Who knows how long they've been there as I haven't left my own place in a few days. This time they weren't in the door for long after I saw them but I left home in the afternoon, my guess is they were in there since last night at least.

I want to be a good neighbor but am to the point where it's not my problem and this guy needs to learn at some point. We live in a pretty good area, but it's beginning to get a bit more rough. I'm just so shocked this has happened so often. I left my keys in the door one time years ago and was so upset when I realized what happened, it never happened again.

So, what would you do?