Take a group of adventurers trying to get the drop on a lich or any other intelligent undead. They decide to use the spell hide from undead to get into position. What they don't know is that the undead is cautious and has something producing an invisibility purge effect, like a lantern of revealing.
The hide from undead spell description says:
Undead cannot see, hear, or smell creatures warded by this spell.
But it's not listed as being invisible, and the invisibility spell's school is actually illusion (glamer) whereas hide from undead's is abjuration.
Would hide from undead keep the adventurers from being seen? Or would the invisibility purge effect make them visible?