Invisibility Alarm

Abjuration
Level: Alchemist* 1, Artificer * 1, Bard * 1, Hunter * 1, Inquisitor * 1, Ranger * 1, Sorcerer */Wizard * 1
Components: V, S, F/DF (a tiny bell and a piece of very fine silver wire)
Casting Time: 1 Standard Action
Range: close (25 ft. + 5 ft./2 levels)
Area: 20-ft.-radius emanation, centered on a point in space
Duration: 10 minutes/level
Saving Throw: None
Spell Resistance: no
  This spell functions as alarm, except it is triggered only by Invisible creatures entering or leaving the warded area.
  The spell has a different mental alarm depending on whether the Invisible creature is entering or leaving the area. This spell does not aid you in sensing or pinpointing the location of an Invisible creature within the spell's area.
  Invisibility alarm can be made permanent with a permanency spell by a caster of 9th level or higher at a cost of 2,500 gp.