The simple answer to this is the question "Who would be the rogue?"
It's not Redcloak. It's not Xykon. I really doubt it's MitD. Oona might have some tracking skills but she's some kind of beastmaster class, probably better at setting traps than finding them.
Logically speaking a high-level party should have a rogue but if they don't have one they don't have one.
Also Redcloak only has one eye. Might affect his ability to spot things.
My theory would be that if you trigger the trap unintentionally you don't notice it. It's supposed to be an invisible portal.
Haley didn't activate the trap, she used a method of disarming that resulted in the magic trap misfiring and exploding. It's like when Blackwing intentionally miscast Vaarsuvius's scroll to cause an explosion (#1020).
Of course this trap is significantly higher level than Vaarsuvius's scroll and would require more effort to blow up, which is why it requires Haley carefully working on it while the rest of the party discusses how to proceed (she wouldn't have to say that she 'figured it out' if it was as straightforward as triggering it with an inanimate object).
He's looking right at the line and when asked by Haley if he can see it he answers "No." He has a rough idea where it is because Haley pointed at it. Also just because everyone could see the magical glyphs in Draketooth's Dungeon doesn't mean all magic traps are visible.
Basically everything Hyoi said.
I like that last part. It would also explain why Redcloak and Xykon couldn't bypass it even if they used Truesight or whatever to detect the runes. Might have tried dispelling it and dismissed it when it didn't seem to change anything.