Is it possible that the light you see is supposed to indicate a bulb failure? If so, check to make sure someone has not switched bulb sockets around in the tail lights, and check that the wattage is exactly the same in all the bulbs in the brake circuit. Mismatched bulbs can set off a warning light.
The other possibility is more disturbing. Could the light be a brake system failure warning? That would mean an imbalance in pressure between the two brake circuits. If it was not inadvertantly triggered by recent brake work, it could indicate a leak at a wheel or a failing master cylinder.