Quote Originally Posted by D20srd
Your Hide check is opposed by the Spot check of anyone who might see you.

If you are invisible, you gain a +40 bonus on Hide checks if you are immobile, or a +20 bonus on Hide checks if you’re moving.

Typically, your Spot check is opposed by the Hide check of the creature trying not to be seen.

A Spot check result higher than 20 generally lets you become aware of an invisible creature near you, though you can’t actually see it.
I'm going to assuming "generally" here refers to when it isn't actively using the hide skill. Seems like a very reasonable amount of RAI.