[JohnPaulB]

Do spells leave psychic or energy traces that can be noticed by those in the know? Kind of like "I sense there was a spell cast in this room sometime today." This sensing would be on magic that had been active and ended off at least one hour ago but not more than 4 days. It would cover probably 20x20 feet cube.

This sensing could be the result of a meter device that detects the decay of energy particles from magical radiance. Or a litmus paper that turns color depending on the amount of energy that was expended. Or a slightly burnt smell or metallic taste of the sensor when near recently spent psychic energy.

Might it be possible to track these traces if they move? "WizJoe, must have been holding a magic lantern, walks from the front door down into the dungeon where it gets very muddy with other magical happenings."

I think these beings who do the sensing are either using very powerful spells/device or are a type of psi that has been experiencing this type of thing all its life.

Perhaps even like forensic Magic (without the crime part)

Anyway, some ideas.