While the title seems to already anwser the question on it's own (there's no way you can detect something undetectable), should that really be the case?
Let's check three examples:
- Collapsing Floor trap: it looks identical to all the normal floor around it, except putting too much weight on it causes it to collapse.
- Door Trap: a mechanism that triggers when a heavy door is opened, causing a massive explosion that blasts everything within 100 feet to smithereens, possibly launching the door at the party at high speeds.
- Magic Trap: triggered by an Alarm spell, except the material components were eschewed / made invisible, as well as applying a nondetection spell on it to floor detect magic and find traps spells,
Let's say we have a Rogue with 30 Passive Perception, as well as expertise in Arcana and whatever other skills might be useful.
Do they notice the supposedly undetectable floor trap by noticing the floor sag by a tiiiiny bit, or feeling that is starts to give way as soon as they lightly step onto it, quickly withdrawing their foot before it triggers?
Do they notice that the door's movement isn't quite right – actually, they would have no idea how heavy the door is, and as soon as they hear the mehcanism's clicking, it's already too late. Does perception translate into intuition/gut feelin/sixth sense in this situation, or is this trap outright impossible to prevent, not even allowing perception checks or whatnot, making the trapfinder rogue feel useless?
As for the magic trap, is the only way to detect it to have Truesight (not happening before level 20), or even blindsense (still very high level for a relatively mid-to-low-level trap) to somehow feel the invisible arcane runes/silver string – that sounds like a huge stretch.
TL;DR Can a Rogue detect a trap that is physically and magically impossible to notice, if their perception (and arcana) are high enought? Or are some of the more clever traps just impossible to find, making them top choices against intruders in the D&D multiverse?