Before your 12-day trip, was the car driven every day?

You could have something somewhere that's left on drawing draining the battery slowly. Happened to my sister's Saab. She took it to a shop, they hooked up current meter over battery's terminal (after disconnecting it) and pulled each fuse one at a time to see what made the current draw drop to 0 (or close to, you won't get 0.0 with clock, etc.). It turned out to be the cargo area light (in a hatchback, so couldn't see unless you were looking for it), where a nut came loose and it couldn't detected 'closed' position, leaving the light on thinking you still had your head stuck in there.

I think you can leave the battery alone, after recharging it, and put the current meter across each fuse terminals, if you can access it. That's how I would've done. Don't know why the shop did it the way they did. Maybe I'm missing something obvious.