I've had a couple instances of my Jeep's battery being drained after a week or more of inactivity. The sharp technicians at East David Automotive found that the little USB charger shown below was using 30 milliwatts (mW) all by itself, even though it wasn't charging anything. Eliminating that reduced the load the car was pulling when turned off from 70 mW to 40 mW, well inside the normal range. This charger is now stored in the center console and will only be used when actively charging a smartphone.
I'll still have to use a trickle charger when the Jeep isn't used for several weeks, but it should start OK if left alone for a week or two.