dnd 5e – Can a Rogue find an undetectable Trap?

I’ll quote the opening pages of the Player’s Handbook.

In cases where the outcome of an action is uncertain, the Dungeons & Dragons game relies on rolls of a 20-sided die, a d20, to determine success or failure.

If the outcome of an action is certain, then there is no roll or ability check to make to attempt to change the outcome.

Rogues have no special trap-finding sense

While Rogues are traditionally good trap-finders, and they can definitely optimise Perception and Investigation skills to be ludicrously high, rogues have no special features which let them supernaturally detect traps (by the book, anyway). There may be features or spells from other parts of the game which can help detect traps, but not from the rogue class features.

It is up to DM judgement as to whether a ludicrously high result on a Perception check results in nigh-supernatural senses. A DC 30 ability check is the typical DC for a “nearly impossible” task, and what counts as “nearly impossible” as opposed to just plain impossible is up to the DM and the tone and genre they want at their table.

But is the trap really undetectable?

In your cases, you need to consider whether all components of the trap are completely undetectable. In some cases this may be true, such as in the heavy door scenario or the magical alarm. But in some cases the disguise may not be perfect.

To take the collapsing floor as an example, there might be hairline fractures in the floor, or it might sag slightly, or when a single foot or 10-foot pole is placed on it the floor sounds hollow or shifts slightly. These might be very difficult signs to detect, but someone with an astonishingly high Perception should notice them.

Even in the door scenario, it may be possible to notice a glint of the mechanism through a gap around the edge of the door. Trap makers aren’t perfect.

But even if the mechanism of the trap is somehow perfectly obscured, traps do not exist in isolation. Traps will be avoided by the denizens of the dungeon. As such, there should be evidence that the space associated with the trap is unused while nearby spaces are used. The trapped door, on closer inspection, shows no sign of use. The collapsing floor has dust but no footprints. And so on.

Checking for lack of footprints applies to old traps. For very recently created traps, you might instead find evidence that the trap-maker had been about recently. This is perhaps less of a tell-tale sign of a trap, but it’s something.

With a very high Perception, you need to think very carefully about whether there is truly absolutely no possible way to detect even a clue of the trap’s presence, even indirectly.

If you are the DM, you also need to consider why you are designing a literally undetectable trap. If it’s a matter of “that’s what the NPCs would do”, then consider also the possibility than the NPCs might not be flawless trap makers able to remove every single clue of a trap’s existence, and that other factors may indirectly reveal the trap anyway.