c. 1500, "one who tracks or searches," agent noun from verb form of trace (n.1). Meaning "bullet whose course is made visible" is from 1910.
Entries linking to tracer
trace n.1
"track made by passage of a person or thing," c. 1300, from Old French trace "mark, imprint, tracks" (12c.), back-formation from tracier (see trace (v.)). Scientific sense of "indication of minute presence in some chemical compound" is from 1827. Traces "vestiges" is from c. 1400.