tracer

noun

1. a person or thing that traces; specif.,
   1. a person whose work is tracing drawings, designs, etc. on transparent paper
   2. a person whose work is tracing lost or missing articles, persons, etc.
   3. an instrument for tracing designs on cloth, etc.
2. ☆ an inquiry sent out for a letter, package, etc. that is missing in transport
3. ammunition that traces its own course in the air with a trail of smoke or fire, so as to facilitate adjustment of the aim
4. a substance, usually a radioactive isotope, used to follow a chemical process or a complex sequence of biochemical reactions (as in an animal body), to locate diseased cells and tissues, to determine physical properties, etc.
5. a thread of contrasting color woven into or stamped on wire insulation for identification and for aid in tracing a circuit