When I am decelerating hard and really press on the brake pedal, I get a "low brake fluid" warning, despite the fact that the my fluid is topped off. Brake pads and rotors were last checked 9 months ago. What has gone spongy and is causing that fault indicator?

 

Please clarify: has the pedal gone spongy? If so there is air in the lines. But that should not cause the low fluid message.
There is a float in the fluid reservoir, and it sometime sticks or actually sinks. If you use the search function you will find some threads that address this problem.