It takes too long for Steve to realize. After they fight Thanos, after he sees Bucky turn to dust in front of his eyes, they regroup. They gather everyone who's left, and Steve's stomach drops when he realizes it's really not a lot. Natasha stands next to him, sizing everyone else up as well.
"Where's Sam?" he asks her, turning his head to look at her.
Because Sam wouldn't let something as stupid as Thanos kill him. Not after everything.
Except.
Natasha looks at him with a pitying look, and it feels like there's a stone in Steve's throat. It hurts.
He's never felt more painfully alone.