It's not a good idea at all to start mixing different senses to vision-only
thing. I don't recall players having the change of 'seeing' invis foes by
hearing or sniffing them out (not that it's really an issue since mobs are
there to kill you, and will attack on sight).
But if you're invis, you're invis, no reason to say 'yeah that ogre-dog can
smell your pants you haven't washed in a week, so quit whining, it doesn't see
invis but it smells invis!'.
This does bring up an intersting point, if someone is invis you'd think it
would create some kind of distortion you might have the chance to see, so
perhaps players could be granted a small chance to 'sense' invisibility as
well.
Players will be granted the ability to see invisible creatures. It involves a
spell called "see invisible" that just about everyone has. Oh, and some items.