Just had an ephinaphy about this dungeon: if adventurers needed skilled keymaster/scout like Chillack for a dungeon expedition and the dungeon has been around for a number of years, why didn't the previous successful expedition actually mark where the secret passages are, where the switches, and what traps lays ahead on the wall of the dungeon, warning incoming new adventurers of the dangers and shortcut?
The answer is the dungeon cleaners. They would erase any foreign substances and damages to the dungeon architecture and restore it to default mode.
Also, the Mad wizard occassionally changing the floor plan wont help either. But he rarely wander around at that level.