As has been said, Brake light probably comes on when the level is low, as there is some sort of sensor on the lid.
(However, I've also had it come on when the alternator was dying. I'm not sure how or why, but the mechanic said it happens with this type of car, too low voltage from failing alternator and low battery voltage too sends a false signal about the brakes. Just something to keep in mind)
ECM means a fault has been found in the electronic injection system, and you should check for codes in the ECU.