In the OD&D example of play, the party don't enter the, presumably, empty room. They listen at the door and hear nothing, so they go on to the next door, hear monsters, and kill them.
I think the main reason for this is that speed is very important in OD&D, due to wandering monster checks. The 1-in-6 chance that a monsterless room might have hidden treasure is actually not high enough to be worth the risk of random encounters.
Incidentally, in the play example, the players obviously know the rules very well. For instance, they know that boots and cloaks are the only kinds of clothing that can be magical - "do there seem to be any old boots or cloaks among the old clothes in the rubbish pile?" So I would assume they know the chances of a monster-free room containing treasure.
EDIT: The other reason for only entering a room when you've heard monsters might be to gain surprise advantage.