Can anyone explain this behavior?

You are correct. I had a Gen 3 (2010 IV) and the fob didn't have that function.

 

The sensors in the door handles work by sensing the water in your hands. I believe they use a local magnetic field that is calibrated to detect changes when significant water is nearby(like within a few centimeters). When you wash the car, it can trick the sensor into thinking your hand is near the handle. These sensors are separate from the keyfob sensors.

The keyfob sensors detect when the fob approaches the vehicle and begins to wake it up. It also activates the magnetic sensors. When your hand enters the magnetic field while the keyfob is within range, that's what locks/unlocks the door. I believe the magnetic sensors are turned off either when the fob enters the vehicle or when the power button is pressed so the system can't be triggered in a massive downpour.


Personally, I just ignore the locking/unlocking when washing my car. If it really bothers you, just move the fob outside the range of the car until you finish the wash.