If you mean the pad wear sensor, it is a one time use sensor. If it tripped, then you replace the sensor. The light then goes out. If they didn't replace the sensor, that's the problem. If they did, maybe a bad sensor or a poor connection.

If you mean the brake warning light, could be brake fluid level, handbrake, etc.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue