Tracer

noun

1. A compound, element, or isotope used to track the progress or history of a natural process.
2. A round of ammunition for a firearm that contains magnesium or another flammable substance arranged such that it will burn and produce a visible trail when fired in the dark.
3. The act or state of tracking or investigating something.
4. A request to trace the movements of a person or an object, such as a shipment.
5. A person who traces something.