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Worn pads could cause your fluid to be low, thus causing the warning light to come on. You could just add fluid but that wont really fix the problem, plus if you do change the pads and don't bleed the system, you will likely overfill the reservoir when you push the pistons back in. It would be best just to change the pads if they are almost gone.
Oh another thing to check, I know its really simple... but your emergency brake, make sure it isn't slightly engaged.