To enhance the atmosphere of an exploration scene in a game, the GM should be careful never to describe anything that the characters couldn't notice with their normal senses. There may be secret doors, or rats under the floorboards, or giant spiders lurking in the dark corners, but if the players aren't poking around in those areas, they shouldn't notice them.

To further improve the experience, make sure you don't describe anything the players don't specifically say they are looking for. To illustrate:

Player: What do I see?
GM: A room.
Player: Is there a treasure chest?
GM: You don't see any.
Player: A cupboard?
GM: Not that you can see.
Player: Do I see a wardrobe?
GM: Nope.
Player: So there's nothing in here.
GM: I didn't say that.

Transcript

Pete: You're in a long, dark corridor, 10 feet wide, lined with shadowy recesses and slimy, organic-looking remains.
Padmé: Maybe we should go back and find a safer place to land...
Pete: The door slams behind you with a heavy "thud".
Padmé: Okay, never mind. Is there anything alive in here?
Pete: Not that you can see.
Anakin: So there is something there?
Pete: You don't see anything moving.
Padmé: Do we see anything not moving?
Pete: You can't make anything out in the darkness.
Anakin: I ready my laser sword and inch forward, fully alert.
Pete: As you pass the half-way point in the corridor, you hear a leathery, skittering sound behind you.
Anakin: Oh lovely.
Padmé: Half-way? So the fastest way out is to keep going forward...
Pete: So it is.