OK, we just played through this, so I can give a more specific explanation of the situation.
They uncovered the wall safe within which a powerful wizard had kept his backup spellbook. The niche is barely large enough for the book. You swing open the cleverly locked front panel, and THERE is the book, and the Symbol, right in your face.
I decided it just wouldn't be fair to make it the "triggers when looked at" type, though that's probably what the wizard would have done. I guess I'm too wimpy.
They wanted that spellbook! So I told them they would be allowed to attempt to Disable it. How to physically describe the success became immaterial, since the fellow doing the disarm check rolled low. The rest of the party had retreated back to the opposite wall, but that wasn't far enough, and one of them got herself killed.
Then they disabled the Symbol during its 10-minute cooldown.