You will see it, but you will not know it. This is the space hidden from your eyes.
Okay. The hidden compartment is a secret rest area for the crew. It's usually located on wide-body aircraft. On Boeing planes, it's in the nose section. To passengers, it looks like a small closet, but a staircase might be hidden behind it.
These rooms may contain 5 or 6 bunk beds and basic amenities. On long-haul flights, the crew may need rest.