I was contemplating the opening scene of Captain America: Civil War and...never in my life have the words "Sergeant Barnes" hurt me more.
Because looking back on the previous movies, I'm almost certain that Howard was friends with Bucky during his Howling Commandos days. Not only that, but he was there in the aftermath of the mission to capture Zola. He witnessed firsthand the effect the cost of that mission had on Steve and the other Howling Commandos, not to mention that it would have affected him pretty badly as well. There's a definite questioning, shocked and disbelieving tone to his voice when he says "Sergeant Barnes?" There's definitely an "oh shit, you're alive?!?" subtext to it, and no wonder, because it must have been almost impossible for him to wrap his brain around the fact that his friend, the man that for nearly fifty years he'd thought was dead, was standing there in front of him.
What makes that whole situation even more terrible is that there's also wariness and caution in Howard's voice, mixed in with the shock and the disbelief. He can tell that there's something off about Bucky, that he's not quite the Sergeant Barnes he once knew, but he can't quite put his finger on what's wrong. Still, I can't help but wonder if Howard knew what was coming. I can't help but wonder if he realized that he was about to be killed by something that appeared to be the man that he'd once called his friend.