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.
Pathfinder Roleplaying Game Advanced Class Guide © 2014, Paizo Inc.; Authors: Dennis Baker, Ross Byers, Jesse Benner, Savannah Broadway, Jason Bulmahn, Jim Groves, Tim Hitchcock, Tracy Hurley, Jonathan H. Keith, Will McCardell, Dale C. McCoy, Jr., Tom Phillips, Stephen Radney-MacFarland, Thomas M. Reid, Sean K Reynolds, Tork Shaw, Owen K.C. Stephens, and Russ Taylor.