Its the dash brake light. I took off the reservior top a good bit lately (bleeding the brakes), so maybe I did do something to the top. I'll look at it Sunday, as I am about to leave for the weekend.
I've got this same issue. On hard lefts, my brake light comes on. I checked all my pad thickness, and they're fine. The fluid level in the reservoir is between the low and high lines, and I've cleaned the contacts in the floater switch thoroughly, and I still get the light.
My theory to the cause:
The float in the brake reservoir is on the left side. When turning left, the brake fluid sloshes over to the right side due to centripetal force, and the float drops down, engaging the switch. I think the brake fluid sensor is poorly designed, and I don't see a way to avoid this light. If I were to turn the thing 90 degrees, then I'm sure I'd get the light under acceleration or braking.
Are there any real fixes for this? I realize it's just a pointless blinky light, but it annoys me. I think that I might buy a spare reservoir to butcher and add a baffle. I'll put the float in a cylinder that prevents it from moving laterally, and put a small hole in the bottom so that the fluid level in the cylinder can only change very slowly, this way, hard turns will have no effect on it.