A4 Brake System Warning Light

I have a 2000 X Audi A4 1.9 TDi 115 SE. I had a brake pad low warning light come on and changed the brake pads. That is now fine. However since then when i start the car, the exclamation symbol ( ! ) comes up on the dash on start up. It beeps three times and the abs light flashes up once. After 2 seconds it goes out again and doesnt come on again until i next start the car.
It never comes on when driving and doesnt always come on at start up but about 80% of the time.

Any ideas?

I would of thought if it was a live abs fault it would come up while driving.

Plugged in to our diagnostic machine and it reports no codes.

THIS WAS FIXED QUITE EASILY. I READ THAT THE ABS CIRCUIT IS FURTHEST FROM THE BATTERY SO COULD BE CAUSED BY WEAK BATTERY. PUT JUMP PACK ON TO THE CAR BATTERY AND THE LIGHT WENT OUT AFTER STARTING. THEN REPLACED THE BATTERY WITH A HIGH QUALITY NEW ONE AND ITS NEVER BEEN ON SINCE!