Steve couldn't help but watch the careful way Bucky touched Natasha, his fingers skimming softly along her skin, a barely-there touch that probably tickled more than anything else. It wasn't quite cursory, but it was still an obvious example of Bucky doing this because Natasha had asked, not because he wanted to. The tender way he tucked her hair behind her ear, however... that was different. That spoke of history and familiarity, and that came as a shock. Steve hadn't known that Bucky and Natasha were close, and while all of the Victors shared a certain amount in common, this seemed like more. And it made his chest tighten up in an odd way, this new knowledge that there were other people who might mean more to Bucky now than Steve himself did. That there were others who knew better what he needed, knew how to step in and settle him, bring him back to himself.
It was an ugly thought, a line of pondering that Bucky didn't deserve, and Steve forced himself to let it go. He didn't have that kind of claim on Bucky, and besides, if someone had been able to be there for him, when Steve couldn't be, that was a good thing. Even if that person was Natasha Romanoff.
He continued rubbing Bucky's neck as the car pulled to a stop, keeping quiet as he took the keys from Natasha, who was obviously still irritated with him. Keeping quiet and keeping still as Natasha smudged lipstick along his jaw, then made quick work of Bucky's shirt. Then there was no choice but to slide out of the car and make for Natasha's door. Steve tried to keep his shoulders loose, his body relaxed, like he'd been having a good time and was planning to continue the good time once he got inside. Because this was more than just keeping himself out of trouble - Bucky and Natasha were swept up in this, too, and Steve needed to make sure they were going to be okay.
With that thought in mind, Steve unlocked and opened the door that led to the lobby, but stayed in the entryway, visible to anyone who might be watching. And as soon as Natasha and Bucky reached him, he slid his hand around her waist and hooked two fingers in the open spread of Bucky's shirt, tugging the two of them in after him. Once inside, he found he wasn't sure if they needed to keep the act up any longer, and he darted a glance at Natasha to check in, figuring she was best suited to make that call.