I am no electrical expert, but a number of functions keep running even with the ignition off, and that number increases if the doors aren’t locked.
My XK is left for two months at a time when we travel to NZ to visit our daughter. I leave it with the CTEK battery conditioner running, and this has worked perfectly for the last five years. It is perfectly OK to leave it like this, and an an awful lot cheaper (and better for the environment) than buying multiple batteries.
I also value the assurance of knowing that when I go back to the car the battery will be fully charged.