"I took an interest while you were unconscious." Steve explains breezily though it is the truth. One of the scariest moments in the battle for him was realizing that he had no idea how to revive Stark because of the arc reactor. It is really dumb luck they got through that moment.

Steve struggles to find a hole in the other point, but honestly he's right. He had no way of knowing he didn't chuck it out or lose it at some point because he never called. "... I never needed to use it before." He grimaces because it sounds terrible. His significant other was kidnapped, his house blew up, etc. But at the time he felt too guilty to speak to him. It's not something he wants to come out and say without some prompting.