Bucky hated going out in public when he was still a fugitive from the law, but Howard Stark needed his help. He didn't exactly feel like he was in a position to deny him for any number of reasons, some of which he had yet to disclose. But they needed more than two people to cover the portal in case anyone new arrived. Someone else already had, but Bucky hadn't found out who it was yet. He assumed if it had been Steve, Stark would have told him right away. He knew he'd find out in a few hours, anyway, unless the newcomer had decided not to join them at the warehouse.
He lingered by the parking garage, far enough away so that no one working there would notice him, close enough to get to the elevator as soon as he heard the ancient contraption grind back to life. And two hours into his 'shift' that's exactly what it did. He positioned himself near the door, strolling up to it non-chalantly, smoking a cigarette, so anyone might think he was just passing by.
When the doors opened he was standing right in front of them, and there was a guy in a red stretchy get up, pointing a gun at him. "Whoa, buddy. You're gonna hurt someone with that thing," he said. "Maybe even me."