Lexus ES 300 Forum : “the brake light on the dash comes on intermittently ,, what can be wrong?”

I would check the switch for the emergency brake. Maybe its hung up or not fully disengaged.

Is it the ABS light?  It may come on under hard or uneven braking, or there may be an ABS controller issue or it may be like Krez said, a sticky E-brake switch.

Change your pads.  There is enough fluid to run through a new set of pads.  As the pads wear, more fluid enters the system, and the level goes down.  When the light is on, you either need new pads, or you have a brake fluid leak.  I'm thinking you should look at the pads.

Could also be a faulty brake pad wear sensor.