The drain is probably in the order of a few hundred Wh a day, if that. The main battery holds 12 kWh. How many years are you thinking of parking the car? Li-Ion batteries have a very low self-discharge rate too.
Just don't leave the lights on with an empty main battery of something like that.
I left mine in the garage for a month while we were on holidays. It was a couple of bars down from full when I left it, still the same when we got back. So, self discharge is negligible.
Somewhere in the Mitsubishi documentation it states that while in storage, it should be checked every 3 months and if it shows no bars on the indicator, to press the charge button and run the engine until 1 bar shows.
10 days at the airport is not a problem.
The aux battery gets topped up whenever the start button is pressed ready for driving, but not if you press the button without your foot on the brake to just listen to the radio. Also, it gets a charge once a day from the main battery, but that applies only to the model with wi-fi.
