The Beholder is a tiny observation structure that can be built practically anywhere, even on the side of steep cliffs.
It costs -3 energy while uncloaked, and -15 when it's cloaked.
It cannot see cloaked units, but likewise, spy bots/stealth tanks etc. cannot see the camera. If you set a staggered line of them in a gap, any cloaked units moving through it will be revealed when they unwittingly blunder into one.
Beholders are instantly destroyed by a Juno missile.