Theft is still theft though guys, in most cases the culprit is local and if you find one with your reg number on someone elses car i think id take them back and ensure he needs more than center caps when he gets back to his car (like a tow to the garage to inflate his under inflated tyres).
Tony
(honestly im not vindictive but ive had this done to me, the center caps, and replacing them isnt cheap, and i was the ONLY person in the area with a Phase 2 R19 16v but there was a 5gtt that had 2 of these alloys fitted..... work that one out!)