Shortly after breaking into Hogan's room, Colonel Backscheider is facing the camera while his men are searching Hogan's room. Around the time when Backscheider asks, "You wanted a call from the Gestapo?" the stage lights reflect off the lenses of Backscheider's glasses. The fact that the reflection covers the entire surface of the lenses indicates the lenses are flat on the front side. When Backscheider turns to talk to Hogan, the lenses are completely flat on both sides, and the view of the louvered doors through the lenses is not distorted, indicating the lenses are props (meaning the glasses are merely for show).
Shortly after breaking into Hogan's room, Colonel Backscheider is facing the camera while his men are searching Hogan's room. Around the time when Backscheider asks, "You wanted a call from the Gestapo?" the stage lights reflect off the lenses of Backscheider's glasses.
After Hogan and LeBeau go to Paris, Captain Gruber increases security at Stalag 13, including nightly bed checks. There is no way that Hogan, as the ranking prisoner of war, would not be found missing, causing Gruber to raise an alarm.