It probably wasn't a Mod Stu. It was probably a glitch Zep/FFA game. You were most likely allied and spawned at the German base when the Zep was hitting it. If you shoot the Zep as a "friendly" plane now the game will give you the auto 5 min. ban. same as shooting friendly planes.

I got in one of these the other day on the German side but spawned at the Allied base right behind the Zep. I killed the thing.