I'm designing an story arc right now that involves a creature that can "sniff out" vis. The creature is not the point of the story, but serves as a plot hook for something greater, so I don't care too much about the mechanics of how it finds the vis.
A series of random, gruesome murders throughout the characters' city (it's an urban covenant) will get the characters out of their labs...
The players will hopefully take notice of the reports that the creature makes a point of eating one object off each victim:
-- A sailor's "Lucky Hook Pendant" that he carved from the shell of an albino tortoise.
-- Holy Oil that the bishop used to anoint the Templars when they went off on Crusade
-- A sergeant's magic sword
-- Olives from an herbalists shop, that were plucked from a 1000 year old tree on the Solstice
These are off the top of my head. I'm looking for other minor magic items or vis sources that the creature would go after throughout the city.