It was probably a sable, it is actually the dominant color. My Kenya is a faded, patterned (meaning she has a "saddle" and "mask") red sable. There really is no true color distinction among sables (a sable is a sable regardless of how dark or whether it is more gray, black, or red), but they are often refered to by color. GSDs are either solid (black or white), bi-color (all black except the feet), sable, or the black and red/tan. There is no "rare" color/pattern and people who breed for certain colors or charge more for certain colors are considered unethical. In general you want the dog to have strong pigment, which means a very dark face, but it doesn't matter which pattern the dog has. There are a few dogs that have "liver" (bluish-gray) or "red" instead of black and this is a fault because the "leather" (the nose and the feet pads) must be black so these colors are disqualified and not bred. Some people consider them the "rare" colors but that is only because they are a fault so all the good breeders are not breedin them, making them appear more rare.
I'm assuming the one you saw was more like one of these?
Red sable
lighter red sable (like mine in the avatar and sig)
black sable
gray sable
The reason the police dogs are typically these colors are mainly because police use European working line dogs, and working line trainers and breeders care more about the dog's ability's than his color, so since sable is genetically dominant that's what they usually get. Also, a very dark sable dog with a good mask is very imposing, visually, and the vast majority of times the police K9 is called, he is only providing visual deterence which means he is barking and simply *looking* aggressive. It is rare that the dog has to actually engage in a chase and take-down. The black and red/tan that is common in show lines is viewed by the public more as the classic "pet" type dog. For example, which would you be more scared of in a back alley, Dodger or Ozzy Barnero?
This is the same reason the police don't use whites. For one, the working and narc people don't breed whites and for two they are just not as scary looking.