[Talan Va'lash]

What Ozy said is a good guide.

If you're in hlint you're not going to respond to the aural clues that an invisible person is approaching because theres no reason that footsteps some distance behind you would be odd. There are tons of people walking throughout hilnt (more than you can see IG as commoners arent represented visually IG even though they're there.)

I just referenced the (3.5) invisibility spell description and there is no provision for a spot check at all except under extenuating circumstances such as stepping into a puddle (which would make a splash) or if your presence is detectable through your manipulation of the objects around you.