this light, the BRAKE light, will also come on when the level of fluid in the brake fluid reservoir is low. Now, even though your actual level may be above the MIN mark, it may be low enought to trigger the float switch.
I would add a little new DOT 3 brake fluid to this reservoir. This should prevent the light from illuminating.
Toyota and Lexus have very sensitive float switches.
Be very careful of this fluid as it will strip paint off of any metal.
steviej