All right, This is why you should always get a police report on stolen items.
Find the SN. If it was ever hydro'ed, either the shop or the hydro facility keep those records, they have to. If it due for hydro, there are a limited number of places that do hydros. With a police report, it takes very little pressure to have these places look for the tank when it comes in for service. Another option is to photograph the tank, ( good for insurance purposes) list the serial #, and inform all the shops in the area that it is stolen. Most will keep an eye out. I've seen seven tanks recovered this way. Once, in LA, a guy I knew had items stolen in Oregon, and recovered locally on a dive charter. He kept an eye out, and a copy of the police report in a baggie in his dive bag. Saw his tank, informed the captain, captain called the police and arrested the guy as he got off the boat, possession of stolen property. Managed to recover most of his (trashed) gear.
How weird is that? We live a small community, it pays to pay attention.