I'm voting No based on the information we have. If the rogue didn't take 20, that vote would change.
"The trap being attached to another area" doesn't work with the rules as written. If pulling the lever activates a trap, trapfinding would let the rogue spot the trap, even if the mechanism that spring the trap is elsewhere. That is, as far as I know, the way trapfinding works. If stepping on a pressure plate gets me shot with a dart, I don't need to use trapfinding on the spot where the dart comes from. I need to use trapfinding on the plate.
If the party missed all kinds of potential evidence and warning signs, that's a different story. But based on the information we've been given here, I think it's fair to say that it's unfair.