A Hunter-Seeker droid was a HoverTech suicide droid used by all factions involved in the Second Tiberium War and the Firestorm Crisis, whose sole role is detonating itself along with an enemy structure or unit.
Once launched, it automatically locates and attacks the enemy base, even if it is out of sight or stealthed. Thus, a commander can easily find an enemy base by looking at its course. The droid moves at extreme speeds and is impossible to destroy en route.
It is exceptionally powerful, able to destroy any unit or structure in one attack - not even the Mammoth Mk. II or the cyborg commando can survive the detonation. However, the droid does not reveal new territory and cannot aim. Therefore it randomly goes for a target.
The hunter seeker droid wasn't used after the war.