tracer

noun

• a person or thing that traces something or by which something may be traced, in particular.
• a bullet or shell whose course is made visible in flight by a trail of flames or smoke, used to assist in aiming.
• a substance introduced into a biological organism or other system so that its subsequent distribution can be readily followed from its color, fluorescence, radioactivity, or other distinctive property.
• a device that transmits a signal and so can be located when attached to a moving vehicle or other object.