They are prepared for that eventuality at the airport carparks, it happens a lot more often than you think.
10-15 years ago cars had a lot less electronics that was running all the time so drain was slightly lower, back then all you had was the alarm and in most cases that did not have ultrasonic sensors, just simple door opened sensors, so a lot less power was needed for it.
Now you get ultrasonic sensors as standard, keyless entry, body computer that is awake all the time to give you things like folow-me-home headlights. 300mA is actually not a lot it's less than 4W, that is less than your smarphone is using.
You can get a small solar panel and leave it connected when you leave the car for longer - it will be enough to keep the battery topped up.