I'm planning a campaign in which the players stop a goblinoid host led by a wizard. The plan is for the players to help stop the host along with other conscripted adventurers. Along the way, however, the PCs will encounter goblins who were made into sorcerers intentionally as an experiment in creating sorcerers. I want to hint to the players that the goblins were sorcerers by design rather than chance by leaving a magic sensor for the players to find, like an arcane eye spell in its sensory abilities. The problem is that it may not occur to the players to cast detect magic to find a sensor, so I'm trying to find a sensor that would be visible and easy for the players to "stumble upon" after the fight with the goblin sorcerer. Does anyone know of a good magical sensor that fits what I'm looking for?