Most theifs steal shit from areas they know (brother is a career criminal) because they know the traffic and such. You should go back on a day off, park near the same spot, make it look like you have expensive stuff in your car and see if they buy the bait. Take a friend or two with you and knock them out, drag them to an empty spot, beat the shit out of them, steal their cloths and then duct tape them to a road sign.