I think it's accurate to say that you will find what you're looking for, and you will not see things you're not looking for -- even if they're really, really "obvious".Exploring a dungeon falls into the normal circumstance. The brain is not looking for anything hyper-specific, and thus generally sees obvious threats.
For instance, if you're creeping down an underground corridor, yes, you will tend to spot a monster that jumps out at you. On the other hand, if you're, say, jumping from platform to platform, avoiding a diabolical trap, or fighting a roper that has grabbed your companion, you might completely miss the goblin archers arraying themselves on the balconies above you. In fact, you might miss the ogre coming in the door.