The same thing would apply to doors, lifts, filing cabinets, ventilation ducts, etc. all being opened / activated / etc. by unseen ghosts.
An easy way for the mechanics to handle this would be that any time such a "clue" is observed by an NPC, treat it as though a security camera went red on the scene. That way a flood of NPCs will converge on the clue location, mill about, and then wander off if none of them detect or Bump! the stealthed character. This already happens if you let of a TNT within sight of NPCs but don't manage to damage any of them - they wander over to see if there's anything to see.
