Oracle Text

Equipped creature gets +2/+1 and has "T, Unattach Heartseeker: Destroy target creature."

Equip 5 (5: Attach to target creature you control. Equip only as a sorcery. This card enters the battlefield unattached and stays on the battlefield if the creature leaves.)

Card Rulings

12/1/2004 Heartseeker grants an ability to the equipped creature. The ability can target a creature with protection from artifacts (unless the equipped creature is an artifact creature).
12/1/2004 The equipped creature’s controller chooses whether or not to activate the activated ability. It doesn’t matter who controls Heartseeker.
12/1/2004 “Unattach Heartseeker” means just that — Heartseeker moves off the creature it was equipping and remains on the battlefield.