There was a previous suggestion that traps could make a check to not be noticed, rather than just applying its passive DC against the Passive Perception of the party. So, the trap would roll d20 + whatever, and compare that to the Passive Perceptions to see who would notice it or not.
Ah, gotcha. Glad they dropped that, it doesn't make sense to me for a trap that is already in place! However, it does make sense for the creation of the trap in the first place, to set the appropriate DC level, of course.