[Rygar]

I should say this warrants more in depth investigation. In Burning Woods a tottering gorilla and a moldering gorilla both became indifferent with sneak on, but other undead in the zone such as skeletons and a tatterback ape could see through sneak.

Edit: spawned another moldering gorilla but this one DID see through sneak. Perhaps there is a chance undead can see through sneak? Thus far all skeleton types have 100% seen through but the 'undead with flesh' seem to be inconsistent. Hmm...