You are assumed to constantly be shouting your exact position to everyone nearby, unless you take an Action to stop yourself.
At least, that's what the rules seem to be saying. The DM is encouraged to figure out something that makes more sense.
Or, to be a bit more charitable...
Adventurers (even 1st level) are better than the average person at various "adventuring things", which include having a sense of the flows of combat and the people involved. And thus are better at noticing things that other people miss, like footfalls, air movement, heavy breathing, the stench of sweat and the like. So an adventurer's "combat sense" picks up on someone standing invisibly in a room or standing guard around a corner, but not actually trying to avoid being noticed. Jason Vorhees knows where the kids are... under the bed, in the closet, having sex in the woods, because he is an experienced "adventurer"... but those same kids couldn't notice him standing around the corner from them if their lives depended on it (and as a matter of fact, they do.)