Despite the fact that Sulu knew that Jack and Ianto had sorted things out between themselves, and he'd heard from Ianto that Scotty had sorted things out with them, Sulu still had the feeling that he was leaving something undone. It was probably the fact that since he'd called Scotty, he really hadn't heard anything from his side.
Deciding that it was about time he visited the other man, to see how he was now that things were figured out - and more than that, he figured he should see if Scotty was really as okay with this as he seemed to be. So, after his shift was over and once he'd sorted out his botany projects for the day, he headed down to Scotty's cabin to pay him a visit, hoping he was there as he knocked on the door.
After all, even if he was completely fine with how everything was - that didn't mean they could talk about it.