Don't forget that by drawing unwanted attention, it doesn't have to be attention from the monster they think they're fighting! He's invisible, right? Well, How about he draws unwanted attention in the form of ghosts or spirits? Or monsters that exist in the narrow space between planes where things that are invisible go?
Your player chose to attract unwanted attention. Give it to him.
There were no spirits in that encounter? Well, THERE ARE NOW. The player calls foul and says, "You never said there were spirits here!" You say, "You made me give you unwanted attention! I didn't pick this, you did."
Also, please, on just the simplest level, remember that there are other senses. For example:
- He becomes invisible, but one of the monsters can smell the brimstone on him
- He becomes invisible, but his shoes on the stones sound like nails on a blackboard to a monster
- He becomes invisible, but he power of the spell is glowing like a lantern in the darkvision of one of the monsters
And yes, think off screen. Maybe the unwanted attention is from some rival spellcaster, who suddenly sees someone invading his previously-private dungeon? Or think Tolkien-esque gazing stones that light up like fireworks because of his choice for unwanted attention.
Dungeon World works because of the way that moves feed back into the fiction. If there is a move with no impact on the fiction, something is wrong.