I had always assumed the spider tracer was a gadget invented by Peter Parker, one that sends a signal to some receiver that Spider-Man carries around when he wants to track a tracer.
However, I recently started reading the 1960s Amazing Spider-Man and I have yet to see Spider-Man carrying a receiver. Furthermore, the comics almost seem to imply that these tracers are tied to his spider sense. Hence the question:
How does Spider-Man's spider tracer work?