The 2006 Audi A8 employs two different strategies to detect critical brake pad wear; basically, those are wear sensors for the front (hydraulic calipers) and caliper opening in the rear (electro-mechanical calipers). Either front or rear brakes can trigger the warning light once a specific wear threshold is met.
The front pad replacement procedure is straight forward and has not changed much in years. Resetting the sensors is part of replacing front brake pads. The sensor is integrated into the pad, and is actually worn through by braking action. When the new pads go in, the new embedded sensor is plugged into the car, and off you go. If you just had front brakes put on, either they didn't plug in the sensor, or they didn't use the right pads and didn't have a sensor to plug in.
Rear pad replacement, however, now requires the use of a bi-directional scan tool to perform certain critical operations, one of which is to open the calipers for pad replacement. If you had your rear pads replaced and a mechanical device such as a "C" clamp was used to push the rear caliper pistons into their bore, the rear brake system is probably damaged.
Please let me know if you have additional questions or concerns regarding this issue.