Honestly, Clint wasn't used to visitors who knocked. Loki usually just appeared and freaked the shit out of Clint the first few times, but he was mostly used to people just appearing now. The knock was a little weird. Nat and Jess and Bobbi knocked, but they seldom waited to enter after. Coulson would've just summoned him, Stark wouldn't knock so...
Opening the door, smirking slightly Clint gave Steve a nod, "Hey Cap, 'sup?" A visit from Steve wasn't entirely unprecedented, and Clint had missed a SHIELD training exercise, not an Avenger training day, so obviously, Steve had to have another reason to seek him out.