What about an ele or mesmer spell to dispell unseen enchantments within an area? (very large area).

i.e dispell magic, or reveal hidden X?

Many other gaming environments (at least the old fashioned paper ones I played MANY a year ago now) had this.
It was quite useful, and is a good counter... i.e you have to bring it to be useful, and if there are no stealth characters, it is a wasted skill slot...