It sounded like Steve was talking to someone - the words were muffled tones from the other side of his door, but that deep voice of Captain Perfect was unmistakeable. Guy is so outgoing he could go out and talk to one of those krogan and he'd probably get a hug.

So yeah, Tony's going to investigate.

"Hey Rogers, who the hell are you - " He peeks his head out from the doorway and sees the gun before anything else. "Nice gun." He turns his head to Steve. "I see you're busy meeting the friendly people around here."