Detecting ShowUnit() Units

How can I detect a hidden unit (call ShowUnit()) in an area and remove it?

Assign it to a variable, or add it to a group then just refer to that, I'm not sure how specific a response you want.

I cant use neither of those, btw, the problem got solved, thou now something else is buggying the spell :/

Problem was that the hidden unit didnt get added to the unit group