D&D 5E How would you make a trap not detectable as a trap?

[Eltab]

Put the mechanism of the trap inside the wall, construct a 3" thick lead panel in front of it, and place a normal façade in front of that, aligned correctly with the rest of the wall as the PCs see it.

Any spell will fail to find what is hidden behind the lead. Now you just have to be an artisan in the construction and home decoration industries, to fool mortal senses.

 

[Caliban]

Have an invisible hidden Imp by the trigger mechanism for the trap. The trap is set off whenever the imp decides to pull the lever. And the Imp is 120' away in magical darkness (100' tall ceiling or very large room or long corridor).

 

[Saeviomagy]

Like the title says, how would you make a trap not detectable as a trap?

I would suggest that you stop playing semantics because it's not actually fun.

That said, I would also ask you why you need to do that. My guess is that you are trying to defeat a divination spell, and which divination spell you are trying to defeat is very much material to the answer.

 

[Spookykid]

Make traps look like normal things. Carpet over a pit, bookshelf that releases gas when a certain book is moved, a porticillus with a wheel but the wheel does sets off a trap.