I don’t think it was him. Just from the fact that Seungmin found a keychain that seems to be something important to the other person, I thought it made more sense for it to be Hyesung. I could be wrong, of course.
I thought it was Hyesung because, in chapter 22, the other members asked him to visit Jaeyoung, but he refused, even though he clearly missed him too. I assumed he tried to open the door because he thought the code hadn’t changed — after all, they used to live together — and then ran away because he was embarrassed.
The reason I don’t think it was Jun is because I don’t think someone like him would run away. Quite the opposite, I think he’d stay there and wait just to see Jaeyoung in a miserable state because he seems to like that.

Oh my fucking god baek Jun showed up at jaeyoungs house u guys