Add very light dusting of powder or baking soda or flour on things he could touch that you want to know. If he touches it you’ll be able to tell. Make it super light dusting so it’s not obvious at quick glance.

Put up paper sign With ton of tape blocking use of it marked private so it can’t be opened. Any guest should be fine with not touching anything private.

Put something super small in very distinct way you’d remember so if it’s touched you’d know.

So if you put baking soda all over floor and sign it with your finger so he can’t replace it in room he’s not supposed to be in he can’t ever complain if he sees that as he’s not supposed to be there!