My heart breaks for you because I know how heartwrenching it is to not know where or how they are. I amost went nuts wondering what happened to Leroy when he went missing for three days. It turned out he had gotten himself locked in someones shed or garage and couldn't get out until someone opened the door.
Is Mggie afraid of other people or is she friendly? If she is leary of others then you might ask permission to go call for her in surrounding garages and sheds. If she hears your voice she might respond whereas she hides from others.
I'm praying very hard for you that she comes home. If nothing else I pray that you at least find out what happened to her. The not knowing is the worst part.
I also agree that you should go check the shelter off and on yourself. First hand I know that they have good intentions but a lot gets overlooked. It would be very wise to go and look now and again just in case.
You are in my thoughts and prayers.
Please come home Maggie!!!