See Controls and Indicators → Instrument Lamps → Speedometer Face Indicator Lamps. The security lamp in the speedometer face indicates the status of the security system.

Armed: A lamp that blinks approximately every 3 seconds indicates that the system is armed.
Disarmed: After the system disarms and the ignition is on, the lamp will remain illuminated for approximately four seconds and then turn off.
Service: If the lamp remains lit continuously, see a Harley-Davidson dealer.