Keep it locked in the case, unloaded, and you should be fine. FYI: you can legally open carry with a loaded weapon without a permit here, just make sure to unload before getting in the car also if you plan to visit maine once in awhile.. just apply for the non-resident CCP sometime, I hear it's real easy to get.
to verify the laws check out NRAs website or search http://janus.state.me.us/legis/statutes/
usually takes awhile to find the right law.. but they're all there. :celebrate