There will obviously be some parasitic drain due to car alarm and likely some internal every-once-in-a-while-checks (my guess).
There is also some minor self-discharge due to the nature of chemical batteries (Li-Ion are pretty good here).
One week should be no problem at all. I would not expect the above items to be noticeable with respect to range when you drive home. I have kept my car sitting for over a week without charging and it did not appear to have an effect on range for that charge.
At 20 miles, you should have no problem at all if you start with a fully charged car. Even when the low battery indicator kicks in I believe you should have 10 miles of range available. I have never even seen that indicator personally.
There is warning sound if you leave your headlights on, so at least you won't run into the case of: 1-Lights were on when I left home, 2-When I arrived at destination it was light outside, so I didn't know my headlights were on, 3-Car lacked warning sound so my battery was flat when I returned ( <--- this won't happen
)
TSA will be more of a hassle than your car. ((hmm, was that supposed put me at ease?))
