Locust is correct, yes.
A fairly broad Supernatural Sense that seems, from the text, intended to be automatically successful, at that.
The way I priced it was based off the Sight, which is also automatic but on touch, and includes the Lore-Investigation trapping and, of course, the Sight itself. I completely forgot about Supernatural Sense, which changes things a bit. Also should have been Vampirism, rather then Minor Power, both thematically and because I'm using this for the Jade Court. Revised below.
Based off Blood Drinker and Emotional Vampire, placed one refresh higher due to lack of a forced frenzy and a positive bonus (Detect Magic; Magic Drain is intended to be in line with Taste of Death and Mageslayer is intended to be in line with Drink Blood and Feeding Touch).
Mana Locust (Vampirism; -2)Detect Magic: Within your zone, you may attempt to detect the presence of magic (defined as effects created by or individuals possessing Spellcasting Powers) via an Alertness or Investigation roll. Success provides awareness of the presence of magic, and identifies individuals with Spellcasting Powers if you are already aware of them.
Magic Drain: Whenever an effect created by a Spellcasting Power originates from or targets your zone, its effectiveness is reduced by two shifts.
Mageslayer: Add +1 to close personal combat attacks made against characters possessing Spellcasting Powers.