The sensor can be damaged by brake fluid, which can happen either when it's ejected from the reservoir during hard driving (if it was overfilled) or if some was spilled during a service. Unfortunately, this brake fluid-sensitive sensor is located directly under the brake fluid reservoir....
Happened to me too once. Now I just ask them to be extra careful about spillage and fill level during brake fluid service.
'16 Cayman GT4 (delivery pics
Gone but not forgotten:
'11.75 M3 E92 Le Mans | Black Nov w/ Alum | 6MT (owned 5/2011 - 11/2015)