yeah, Anthony Beevor wrote it in his awesome Stalingrad book.
Germans troops stuck in Stalingrad feared maybe Stukas raids as much as Russians cause of friendly casualties.
They used to lay flat on the ground towards the sky big red/black nazi flags to get spotted by friendly Stukas. And as we know, Stalingrad was a battle of room to room, so Russians used those flags to, trying to pass themsleves for German units.