There are many different color and light combinations of a trackunit to indicate the status and health of the unit.
The color combinations can tell you if the device has power, has a GPS position and connection to a mobile network.
 

No light in LED
No power supply connection to the device.

Red flashing light and constant green light in LED
Mobile network is healthy and GPS has valid satellite position.
This is the sign of a healthy unit!

Constant red light and no green light in LED
Power supply is healthy, but no mobile network GPS has no satellite position.

Red flashing light and no green light in LED
Mobile network is healthy, GPS has no satellite position.

Constant red and green light in LED
Power supply is healthy, but no mobile network, GPS has valid satellite position