Forester security light flashing

02 Forester with the factory security system. Had this vehicle since new. When armed the security light, on the dash, flashes slowly. About once every
2 seconds. It has just started flashing very fast about 2 flashes per second. It only does this when engine is not on. Arm/disarm has no effect on the light. Search though the owners manual and it is not mentioned. What is causing this sudden change? TIA