Security alarm was likely triggered by something.. Cat in the garage?
After it goes off it is supposed to stop after a short time, waiting for another "event".
The security indicator light always blinks - It's supposed to act as a theft deterrent.
If your vehicle has the optional Shock Sensor unit installed that will trigger the car's standard security alarm (horn honking, lights flashing) behavior too when it detects anything causing motion/bumping into the car. The honking and flashing lights will stop after a while and the system resets itself and the security light on dash will be blinking as others have said.