The trouble, I guess, is that the spell effect of a successful spell is for the trap to simply vanish. How do you tell if something vanished that you couldn't tell was even there?

ADOM does this thing in more than one place, where situations that should be indistinguishable from each other get subtle tells in the in-game text (such as doors not budging or not "at all" budging, feet hurting or "really" hurting when you kick walls, so there's a precedent for making the successful disarming of an unknown trap something slightly different rather than nothing at all. "You listen closely to the sound of silence", perhaps? "You are comforted by the sound of silence"?

However, the thing here is that even if you don't know there's a trap on a door, if you cast the Dispel Trap spell on it, 'You hear a clicking sound'. It's quite a distinctive trap removal noise even if you didn't know it was there in the first place. So I'm in agreement with this being a bug; a message was forgotten somewhere. I've thought about it my own self a couple times.

Floor traps do vanish without a sound, though. They don't click. Whether it was an oversight or not and whether how it's currently handled is how Thomas thinks it's best handled is not for us to answer.

Which is why putting in a report isn't a bad thing. :)

After all, it was only through (totally unrelated) bug reports that we discovered Khelevaster wasn't *supposed* to be made invisible in the village when he was rescued!

I think it would be OK if the message were "You listen to the sound of silence" as in the no trap case. But there should be a message.