Now this will sound odd but this experience happened to me a few months back. I had noticed a few posts like this before so I thought it was either my pads or the reservoir that was the problem, even perhaps something to do with the clutch.
I checked the fluid level and it was fine. It steadily got worse and worse, to the extent that the light would come on even mildly accelerating up a steep hill.
Anyway, I took the car into the shop to have the pads checked - the result? "Plenty of life left in them." Anyway, a couple of weeks later it went in for a basic service, i.e. an oil change, and ever since then the light has stayed off!
Dunno why but I'm pleased its gone...