Undead are my most favorite opponent, to be used against my players' characters.
They have learned that undead have several senses above normal.
Detect life: range 100'. It is some sort of connection to the shadow plane, where the living shine as bright lights (and elves shine so bright that some undead would avoid them). I don't like a long range here because I don't like zombies and skeletons to trigger on every passerby. Also it works much like spell in that it is blocked by 1 foot of stone, 3 foot of wood and 1" of metal.
Sight, hearing: range as race the undead originally was. It is like the way it was in life, except undead also have dark vision up to their former range and it is more likely that thieves and rogues have better developed senses, than others.
Taste, touch, smell, temperature sense, balance: For corporeal undead these senses exist and would be like their former living self's senses. Incorporeal undead just don't have them.
I would like to note that I like to spice up my games with such stuff as blind ghosts (just relying on the detect life ability) where they can't see, but have massively increased range for the detect life. Also I like the headless corpse (no sight, hearing, smell and taste) but increased other senses and the composite frankenstein, who has like elvish eyes, dog's nose, elephant's ears, etc.
On a side note: Demons in my game (to make them more terrifying) have "ultra vision" meaning they get all types of vision: Dark vision, Infra vision, Aura vision (this detects living and magical auras, taking care of those pesky invisible beings and illusions), up to 100' unless its a really powerful demon.
Shamble On!