Mistborn is a fantasy book about people with magical abilities called allomancers.
Allomancers "burn" (metabolize metals in the body for magical powers) ingested metals, thereby enhancing various physical and mental capacities.
A bronze allomancer is referred to as a seeker. An Allomancer burning Bronze is able to determine if someone else is using Allomancy in the immediate area. With practice, an Allomancer can determine the location of the other Allomancer, which metal the other Allomancer is burning, and to what extent the other Allomancer is burning his or her metal. An exceedingly powerful Allomancer is able to pierce the "invisibility" of copperclouds.