Firefly (RN-240) - The red LED is a configuration timer indicator. It will illuminate when data is received over bluetooth while the configuration timer is active.
The Firefly has a configuration timer as a security feature. It designates how long the user is able to access the configuration from the serial port after the unit is powered on. When the timer runs down the config will be inaccessible until after the unit is power-cycled. The default time is 60 seconds.
Firebat (RN-270) - The red LED indicates that external power is being supplied to the unit.