With this much argument I'm guessing this is basically down to house rules. However PHB2 says this about Invisible Creatures and Stealth: If an invisible creature is hidden from you, you can neither hear nor see it, and you have to guess what space it occupies. If an invisible creature is not hidden from you, you can hear it or sense some other sign of its presence and therefore know what space it occupies, although you still can’t see it. [Remember that PHB2 also says being hidden makes you effectively invisible to your enemy (p. 222)]
This seems to pretty strongly state that unless a creature is hidden from you, you definitely know where it is—even if you can't see it. It seems to me a reasonable abstraction to let the player know where the creature is even though they can't target it, unless the creature specifically hides itself.
This seems to pretty strongly state that unless a creature is hidden from you, you definitely know where it is—even if you can't see it. It seems to me a reasonable abstraction to let the player know where the creature is even though they can't target it, unless the creature specifically hides itself.