Visible crew/equipment: The first time Hilts (Steve McQueen) is locked up, you can clearly see the edge of the wall, and to the left a stage hand moving. (00:23:25)
Visible crew/equipment: In the scene where everyone is lining up to receive the moonshine that the Americans made you can see a crew member on the left hand side. He is wearing a red baseball cap and modern clothes. He is motioning for the extras to go into the main shot to get the alcohol. (Widescreen edition).
Visible crew/equipment: Late in the film, when the two escapees are boarding a bus, there's a studio lamp on a scissor-lift in plain sight at the left side of the screen. (02:37:00)
Visible crew/equipment: In the scene where the POW's are lined up outside the next day after the escape, several large movie lamps are visible on the left side (widescreen DVD version only - at 2:05:51 to 2:06:16 and again at 2:06:36 to 2:06:40). (02:05:50 - 02:06:35)
Answer: .And, just to add to the previous answer: even if he could speak conversational German, he would likely do so with a very strong American accent (as he does when he speaks the few words to the Commandant earlier), so the guard would have picked up on that right away, anyway.