delericho said:
It is highly unlikely that the original builders of this dungeon placed only this one trap in the whole complex.
I agree.
It is further extremely unlikely that this trap is so significantly more difficult than any of the others that they placed.
Again, I agree.
However, your reasoning has one fatal flaw. You're assuming the PCs have explored the
entire dungeon when they encounter this trap. That's an inappropriate assumption because it's obvious that they have not yet explored at least one portion of the dungeon, the portion that lies beyond the secret door. Perhaps the secret door marks the beginning of the trap laden portion of the dungeon. Consider, for example, the following scenario.
Acerak constructs a a dungeon headquarters for himself as a young, wannabe demi-lich. The "front rooms" (only a small portion of this dungeon) are constructed with the convenience of his servants in mind. He places no traps in these areas because his servants will need to move about freely to do their work and it's cheaper to buy new slaves than it is to construct effective traps anyway. However, at the entrance to his private rooms Acerak constructs a very expensive trap, it is very nearly undetectable and seriously deadly to everyone but him. He does so to prevent both his servants and his enemies from penetrating into his domicile (understanding the evil nature of his servants, he knows he must protect himself no matter how much he pays them or how many members of their family he holds hostage). He places it on a lever because he wants to kill only those who actually attempt to enter his area, not just anyone who enters the room. His servants know that if they require his attention they may enter the room and wait safely for him to appear, but shouldn't pull the lever. Everything beyond the secret door is filled with deadly traps that are tuned to leave Acerak and Acerak alone unharmed.
Eventually, Acerak shuffles off his mortal coil and takes up residence in his tomb as a demilich. His servants wander off and his HQ lies abandoned. Several generations of squatters make use of the front area of his dungeon (one displacing the next in various power struggles between humanoid tribes, evil cultists, etc.) but none have the means to get past his fiendish trap and enter his private sanctum.
Then the PCs happen along. They clear out the "safe" servants area of Acerak's abandoned dungeon. The portion which was constructed with no traps and, like many of the previous occupants before them, encounter the trapped lever and lose one of their number before discovering that the trapless nature of the rest of the dungeon ends in this room.
IMO, a perfectly reasonable and common setup for a D&D dungeon that fully explains the presence of a very deadly trap at one point of the dungeon even though the portion of the dungeon the PCs have already explored contained no traps or traps of a very different nature.
Then the inhabitants would have marked the room somehow (perhaps a honking big skull and crossbones in red paint?) to remind themselves not to pull the lever by mistake or when drunk.
Maybe if your dungeon is populated by the Swiss that would be true. I wasn't aware that Orc or Kobold society had advanced far enough in the D&D world that they had developed OSHA inspectors.

IMO the chaotic humanoid races are no more likely to leave warning signs for others than they are to establish a Social Security system. Chaotic humanoids are self-centered and uncaring by nature. When an Orc sees his companion get dusted by the trap his thoughts are "Now I know never to pull that lever." not "Poor Og, I'll miss him. I'd better put up a warning sign before any of my other friends die." IMO most Orcs who saw a drunken companion pull the lever on a dare and turn to dust would laugh their a***s off because they're selfish and evil and that's just how Orcs are.
And we're both assuming that the dungeon has been populated by sentient humanoid beings. If the dungeon is known to the locals as the Dungeon of Skeletons and Slimes I think a big warning sign is highly unlikely (unless Pedro the Pudding has developed the ability to write "Cuidado" with his own slime trail

).
And, finally, I refer you again to my post at the bottom of page 12, which explains at length that only a damn fool would build this trap in this location.
Which is all subjective opinion based on (what I would call) a number of unfounded assumptions. If my scenario above doesn't convince you that the trap is, in fact, entirely reasonable under certain circumstances, then Raven Crowking does an excellent job of explaining a number of different, additional, reasons why the trap not only makes sense but is, in fact, fiendishly clever.