I have seen this maybe 8-10 times, but only ever on Stormfront. Me and my teammates all run around circling the guy, shooting at thin air, lunge knifing but not injuring him (as though the game detects an enemy but can't go through with the kill), as he slashes back seemingly like "WTF bros?". Then he runs off towards the enemy team and we all go on our way like nothing ever happened. I get aim assist over the mystery player like any other enemy.
I'm thinking maybe it's a loading error for a player joining a session in progress, as they never appear at the start of a game, but only ever for the Stormfront map...unless any of you have seen otherwise.