The "ignition off drain" issue is so ridiculous, none of the industrial electronics those vehicles are made on would lose memory if left off for an extended period of time. The welding control I supplied to all of the GM Truck assembly plants could be left off power for four years, without battery backup and would not lose any programming or stored values, and that was in 1987!
So until they learn how to do this, you can put a battery maintainer under the hood for $24 and plug it in if you will not be using the vehicle for a few days.
Eric O of South Main Auto in NY can track the drain down. One I recently read about was due to a 3g telephone circuit that would search for a 3g tower every few minutes (They have been replaced by newer systems) and wasting electricity continuously. The fuse was pulled on that one because even GM couldn't fix it.