Battery

Test alternator output and parasitic drain.

also check your battery it may have a bad cell

My 2006 Subaru Legacy 2.5 GT Limited goes stone dead if I don't drive it for 2 or 3 days. Previous Subaru had same problem. Subaru says everything checks out fine, battery, alternator, etc. What they won't admit, and what I've finally figured out, is that the electrical system wasn't designed for very light use, or no driving for a few days, while the security circuits, etc drain the battery slowly until it's dead. I have to keep a booster battery in this car, which I love despite its electrical failings.

I would seriously do a parasitic drain test on this vehicle. YouTube it and do your own test. This is not normal. If it did not do this early in its life then something wrong is going on. The test is easy. You need a voltmeter.