What do you mean by "started coming on"? Does it go off at times? If so, can you identify when? The red warning light turns on when the parking brake is on, the brake fluid is low, or there's a leak in half of the system and one part can't build pressure. Start by checking the level in the reservoir. If it is real low, suspect a leak. Your mechanic had no way of foreseeing that. If it is low enough to turn on the light but not almost empty, that can be in response to worn front brake pads but they have to be so badly worn that they would have found that during an inspection. Conscientious mechanics will never add brake fluid when they top off the other fluids during routine service. They understand that to install new front brake pads will result in a lot of brake fluid getting pushed back up to the reservoir and it will be full again. If someone previously added, it will run out making a mess on the floor and it will eat paint off the car if it's not washed off.