Automatic uses minimal power from your car while it is off, so it should be fine to leave Automatic plugged into your car while you are away. It really depends on your vehicle and the health of your battery, as well as how many days you will be away. For more information on power consumption while your car is off, please visit the following FAQ.
Of course, you can always unplug the adapter if you'd like and plug it back in upon your return and shouldn't have any issues.*
I hope this helps, Bob. Safe travels (well, if this question is related to a planned trip) :D
*If you don't drive your vehicle for an extended period of time, though, you may run into connection issues between your phone and the adapter. For more information, see this FAQ.