There are two possibilities, either one each on a front and rear or one each on both fronts. The base W210 was the latter.

The pads are always drilled for sensors because many different models use them; what matters is whether the socket is there so you could plug in the sensor.

And you're right, if no sensor is in place, the car won't know it. It's a single wire system that uses the rotor to close the circuit when the pad wears far enough.

A good dealer would not miss it during a brake job; if the socket was there they'd install a sensor. More likely they didn't do the work and just had the car on the lot. A normal PPI wouldn't normally look for a missing sensor, either.

Hope that helps.