Not so.
The 1e DMG has rules that tell you how long it takes to search a given area, and how often wandering monsters occur (unless the module specifies otherwise). If a module has a time limit, you can determine exactly what percentage of the area can be searched within that limit. If the module does not specify that there are no wandering monsters, you can determine roughly how may additional encounters prolonged searching will cause, and what the average effects of those encounters will be.
A more in-depth analysis of any 1e module will show that the odds of finding all the treasure in most modules (while succeeding/surviving) is virtually nil, so long as the DM uses the searching times in the rules, and makes wandering monster checks in accordance to the rules/module text.
RC
So you are saying all this treasure was in the adventure, but the pcs were not supposed to find it?
Furthermore, given that most of the treasure was usually carried by defeated foes or located in treasure piles/chests, PCs loose out little (10-20%) if they do not spend time searching the dungeon.
Finally, nothing really keeps players from coming back and doing a thorough search after clearing out the section of the dungeon, assuming that wandering monsters actually have to come from somewhere and don't just pop out of empty air.
Is this based on a "feeling", or have you actually examined the modules to come up with those numbers?
The 1e DMG has some discussion on monster populations, how areas re-fill, and how monsters react to PC incursions. A monster lair that is severely damaged, but that manages to stave off the PCs, might not be there when the PCs get back after licking thier wounds. Healing times in 1e play a major factor in this. If you look at, say, Keep on the Borderlands, and read Gary Gygax's notes on how the various humanoids deal with PC incursions, it is quite clear that failure to wipe out a group might mean that the survivors -- and the treasure -- are not there when the PCs come back.
Likewise, there is a dragon hoard example in the DMG that spotlights how, if the players kill the monsters and leave, others are very likely to come in and take what is left.
Of course, this assumes a game philosophy where the PCs are not special snowflakes, where the value of treasure (or encounter) is in its potential whether found or not, where wandering monsters are not considered unfun, where the treasure doesn't follow the PCs around until they pick it up in neat parcels, and where success and failure are determined by game play rather than by meticulously balanced rules that mandate certain types of success (wealth/treasure/magic) go hand-in-hand with other types of success (levels).
TSR-D&D was, from a game design philosophy standpoint, a very different animal than WotC-D&D. A large amount of treasure was available because it was expected that much of it would be left behind.
RC
I was not assuming the PCs would come back for treasure, I was assuming a scenario where every threat had fled or been killed by the players. At that point the player have ample time to tear the dungeon stone from stone to find every piece of treasure they overlooked before. If we take the earlier mentioned moathouse as an example, what is keeping the players from finding every bauble once they have killed all the cultists?
I would argue that TSR had very little design philosophy other than "that sounds fun" and most philosophy that is now ascribed to them is projected from our vantage point of 30 years game evolution. But that is a different topic.
Items were placed, in many cases, to reward "good play", or to allow for the potential of being found. This isn't so different from the inclusion of "secret areas" that might, or might not, be found by any given group of adventurers. In the 1e era, these things were considered hallmarks of good design.
Hmmmm.....possibly (1) other events in the campaign world, (2) more cultists coming from the Temple of Elemental Evil, and (3) more interesting areas to explore. Also, once the moathouse has been emptied, the agents in the Village of Hommlet would certainly move to examine what remained.......
EDIT: I should also note that, within the context of 1e at least, it wasn't intended that the DM tell the players where there were no more threats in an area, or when they had found everything/missed something. Several modules include reinforcements, for example, that arrive after the PC's initial foray(s). Players who assume that they have killed all the opposition and begin tearing the dungeon apart stone by stone may well be in for a rude awakening.
I would argue that reading the 1e DMG could (for many people) easily counter that argument.
RC