I guarantee you it's someone you know... I've had enough stuff stolen and friends I know of have had enough stuff stolen to know that barely anything is random... Just think long and hard, how random it'd have to be for someone to happen upon your open garage to find an unsecured shotgun. Someone knew it was there. Either they saw you roll in and put it there or they saw it there before, or someone told them about it. Could even be a relative that took it, either way, It's someone that can be linked back to you in some way.