M response was so perfect and solves the problem once and for all*, so I thought I'd repost it here:
Suppose there is no light source, just infravision, and no undead. Do the adventurers run into walls?You don't really need to get more technical than that. I do, however, think I would halve the encounter distances for wandering undead, just on the grounds that they stand out less from their surroundings.
If yes: they can't see undead, either.
If not: they can see the undead.
* Yeah, right.