If you point out something on a wall, it’s going to draw attention to it. “You enter a 20×20 room, the walls are wooden and have unlit torches, and there is a piece of paneling that is discolored.” This is going to most likely set off alarms, and your players will scrutinize the heck out of it. So, if you want to catch them with a trap, you either need to hide it really well, or, like the one below, hide it in plain sight, just use a traditional dungeon element in a new way.
An open doorway leads to a long hallway, with brick floor, walls and ceiling.
Type: magical; Perception DC 32; Disable Device 32
Trigger location; Reset None
When a character enters the hallway, gravity changes such that the far end of the hallway, which is 100′ away, is down. The end of the hallway is also spiked. Wall Pit(DC 20 Reflex Save or take 10d6 damage plus 3d6+3 bleed damage and be knocked prone)
Even if they see the end of the hallway, they will probably wonder why they are spikes and close to investigate. They probably recognize this for what it truly is: A Pit.
Remember this when you build your dungeons for other elements. There is nothing saying that pits or tripwires have to be in floors, torches on walls or lights on ceilings. Think about new ways to position some of the basic elements, and you’ll keep your players guessing, even if some of the new elements you throw at them aren’t trapped.