There's probably a parasitic current draw on the battery as the vehicle sits for an extended period of time.
In the past, before vehicles had computers and modules one would remove the negative battery cable and connect a test light between the battery post and cable to see if the light illuminated. And if it did, then fuses and relays would be unplugged until the light went out indicating what circuit was causing the current draw. This method of testing for current draw no longer works because it requires disconnecting the battery.
The computers and modules in todays vehicles can remain on or awake for up to an hour after the vehicle has been turned off. After that time these components then go to sleep or turn off. At least they're suppost to. If the current draw is caused by a computer or module that fails to go to sleep, and when the battery is disconnected, this forces these components to go to sleep. Then when looking for a current draw at the battery none will be seen.
Instead of powering down these systems by disconnecting the battery and causing these components to go to sleep, a parasitic current draw is detected by measuring the voltage drop across the fuses. This indicates what circuit or module is drawing the current as the vehicle sits.
Take the vehicle to someone who knows how to test for a current draw using this method, and I'll bet they'll find out what's causing the battery to go dead.