He was too overwhelmed by everything that is happening, he wasn't thinking straight.
i don't think he "happened to come by". Think about it, if he did at the very least he would've heard them as the seme wasn't making any effort to keep things quiet (even saying no one would show up, which is actually another red flag as why the hell DID he show up other than that he knew they were there?). He doesn't look surprised the entire conversation and when he asks them what they were doing it seems more like he's mocking them because he knows exactly what they were doing as apposed to genuine concern. i think he's going to go after the uke.
And he showed up in the study room right before the seme threw the sheet over the uke, plus it's likely he knew he was hiding there with the way he kept making moves towards the lump on the couch claiming to look for his USB, but the seme diverted him to the USB on the couch. He probably didn't claim it wasn't his because that would look too suspicious so he rolled with it (although it really might have been his). He's been lurking on them for the longest while. Question is why? In my opinion with the way everyone seems to like Hyung I wouldn't be surprised if he was trying to make a move on him by revealing the guy who is at the forefront of the race for Hyung's attention is interested in someone else thus eliminating a contender and creating a chance to move in. Again this is pure speculation on my part, he may even be interested in the uke or seme instead of Hyung but it's definitely someone. We'll have to wait and see.
*USB on the table.
-puts on tinfoil hat- i'm in complete agreement with you. the vibes have been there all along.
Okay, what is with them being questioned why they are hiding out? I know it happens in real life, but instead of changing colors and fearing people will find out what you are up to come up with an excuse. The uke isn't the most social person, he could have just said "I was feeling a bit overwhelmed by the festival and was calming down so I could go back and enjoy it. He just happened to come by and was worried." Now is that so hard? Why is it that they always fret instead of coming up with good excuses. If I was 'caught' in that situation I would have said "I was having a panic attack and he was calming me down."