I agree she looks like a Heritage x production BR, that is what I thought after she got dried off. I still have no idea where she is from. I know there are chickens around me because I can hear roosters crowing but they are not close.
I have heard from at least 2 people in AZ this year who have had birds dumped in their yard. It is more than annoying, I would be very angry if someone did that to me because you just don't know what diseases that bird has.....exposing your flock. I think these people see you have birds and they have one or more they don't want so they just dump them in your yard. Figure you "won't mind"???!! These are the same people who would dump any unwanted dog or cat out in the desert IMO...total jerks.