The parking lever (or emergency brake) activates the rear pads anyway, and I don't believe there are any rear sensors on that model.

Putting it in "P" locks the transmission pawl, and the sensors probably get activated with the slight motion you get when the key rotates until it reaches the pawl.

The sensor just started to come on my ML this week, and so far it only comes on when I pull someplace and "park"...kinda like Joseph_H's situation.

Driving styles dictate pad wear, but on the average, I find I have about 1500-2000 miles of pad life left before the "rivets hit the rotor"...that's a little over a month of daily driving for me...

__________________
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car