Hi Natalie I think people were just trying to help you to prevent this from happening again.
I think the only thing you could do is mentioning it in the review saying “the guest helped themselves to X hours earlier check in without persmission”. Explain to the guest that it bothered you and he shouldnt do it again in private feedback.
Theres no point confronting him while he is there because he might leave a bad review.?! Also, he cant take it back and any excuse will be invalid for example “oh Sorry I didnt know” “oh I didnt check in I just dropped my bag”.
Also block him in case he wanted to book again.