There was no expectation that the PCs would clear the dungeon in one trip or at all, and no expectation of the DM guiding the party towards accumulating all the treasure, perhaps by fudging the secret door rolls.
Besides, there are better ways to guide players toward areas that no one has found yet. If you really want someone to find a sub-level no one has ever explored before, just stick a treasure map somewhere the players will find it that points them in that direction. Or you can introduce an NPC who knows something about that sub-level, but has a price for the information. The possibilities are endless before you have to resort to fudging rolls.