Often, going all the way back to the game system, you will find a chest, and it will have a trap, but then nothing else. I don't consider that an error on the part of the game developers. Other times the trap will be there and the contents will be a healing potion or anti-poison item (if it was a poison trap). Those fiends!

But then we all know about the "empty" treasure chests that aren't even marked in later Quest packs. Clearly an error on the part of someone writing up the quest. Who knows what goes there? As Zargon/Morcar you can either choose to leave it empty, or stuff whatever you like in there, even a really good artifact, some needed gold, etc. your chance to "throw them a bone" (before you destroy them, hahahaha).
Here are a list (for convenience) of the "Empty" chests I've found in the official quests (NA Editions):
**Possible spoilers ahead***
Game System (base) :
Quest 3 (unmarked chest near a pit trap)
left side middle room (two left, third down, if I recall correctly)
Against the Ogre Horde:
Quest 5:
right side, third room down
Quest 7:
left side bottom. though, if I interpret it correctly, it's actually impossible to enter the room & search for treasure, unless you Pass through Rock without using the door, meaning you'd have
to have the Elven Cloak of Passage or the Spell Ring, each acquired in other packs, to be able to escape with anything you'd find! Or put one of those things inside it...
*** (thanks wallydubbs for these!) [Edit: I forgot in the EU rules, as these were originally designed for, you can as the Dwarf or with a toolkit "fix" an already sprung falling block trap after the fact, so under those rules you could escape after searching even if the treasure wasn't a PTR spell scroll or elven cloak!]
Edit: Also, the text DOES say "the chest is empty" so this is not the same as the unmarked chests in other examples.
Wizards of Morcar:
Quest 5:
2 chests, central room
*** (thanks again wallydubbs!)
Barbarian Quest Pack (The Frozen Horror):
Quest 9 (heavily guarded chest with Gargoyle and a bunch of other monsters)
right side, three rooms over, one down
Elf Quest Pack (Mage of the Mirror):
Quest 1 (can't believe I missed this one!). There is an unmarked chest in the room with the fireplace and throne, upper left quadrant.
Quest 2 (under "D" no mention is made of the chest in the room with the Fimir, oops!)
[Edit: I think now the attendants are supposed to have been hiding IN the chest... perhaps it is a large trunk. Or maybe one is in the chest and the other is behind the weapons rack, but either way the lack of any other treasures bugs me.]
Quest 7 (heavily guarded by werewolves and a gargoyle)
right side, second room over
Japanese Game System (to be fair, this is a translation project, in Quest 7, there are three unmarked chests)
in the original version, they are all marked as "empty" in the Quest notes.
Any others I missed?