[AlexanderHowl]

Vibration Sense is a really cheap way to see across planar barriers (the +100% enhancement just allows you to use it on either plane to 'peak' across). It is good enough to detect objects in the way, annoying mortals who might detect a manifestation, annoyed immortals waiting for payback, etc. In the case of spirits wanting to do a little hanky panky, it is probably enough to allow them to find their paramours and be reasonably sure that they are alone.