Ahh, yet more old style Star Wars tom-foolery: Luke Skywalker's breaking into an Imperial base to get some information but the Empire are aware he's on the way and so post guards to capture him as he heads to the computer room.
Thing is, Luke's gone a different way and used a terminal in the kitchen to get the same info, leaving the stormtroopers hanging around in the wrong place.
Luckily, they have state of the art equipment to alert them to their error:
Yep, that appears to be a metal arm that's popped out of the front of his helmet, dangling a message board in front of his face with the word ALERT written on it.
A head's up display, 80's style.