There's a lot of options to try to isolate the problem, just thinking out loud here -
a) Back into the driveway returning from work. See if alarm goes off. Next morning, see if alarm goes off as you drive out.
b) Remove fuse for reverse assistant. (Stupid sensor thinking IR, ultrasound or RF sure-fire sign of break-in)
c) Remove fuse for reverse light. (Stupid motion detector confusing light for potential intruder)
d) Rev it up a little before shifting into reverse. (In case a resonance is the cause)
e) Roll out of the driveway in neutral. (Would be surprised if this triggered anything - if the car's movement was the cause, alarm wouldn't go off immediately upon shifting into reverse, only when it moved)
f) (...)