Is there any "hidden" loot in the game that requires any certain skill to spot or is it always the same loot?

As far as ı remember, there is no such thing. For NWN1, almost every loot is random. For NWN2 they are not. Someone correct me if I am wrong but spot skill is only useful in one quest in the first expension of NWN1.

EDIT:

http://nwn.wikia.com/wiki/Spot

Here you can learn more about Spot skill.

The "Search skill" finds secret doors more easily, and they can sometimes have extra treasure.

I usually play a character with some Rogue and have this skill. Handy for spotting traps as well.

^^^ what PeterScott said. You don't need much Search if you are patient, though. There is a d20 reroll every 6 secs for Search.

However, oft times environments will destroy contents of bashed chests so Open Lock skill or Knock spell can save that loot for your toon. Not technically "hidden" but rather "evasive" loot without the ability to handle locks in some fashion.

Spot skill is used for detection of hidden CREATURES using an opposed Hide vs. Spot check, Occasionally, a designer will select an unlikely skill (like Spot) to reveal a hidden placeable but generally the Search skill is used.

And if you're talking about in NWN2, there's a lot of hidden loot on the overland map of SoZ, and different kinds of loot require different skills to find them. Using a lead character with high spot skills will generally find trap parts or other rogue-related loot, while using a lead character with high spellcraft skills will tend to find scrolls or other caster-related loot, and so on.