I know the Chicago is home-based in Pearl Harbor and I found out on their Facebook page that they've been giving away Christmas gifts to military families recently. It's possible that they may have given that cake away to someone who accidentally left it on your car seat (if your jeep is open) or something.
I wouldn't eat it, either, especially since you don't know where it came from or how long it's been there or if anyone else had access to it.
ETA: If you live in military housing, perhaps one of your neighbors or friends went to some kind of Christmas party. Perhaps you can ask them about it.