Re: thieving bastid muddy funsters.
Have a quick search of eBay for a 156 2.5 V6 in the same colour as yours. Often they will steal a car, then steal plates off an identical car (model, colour, year) so that it looks kosher if the customer does a vehicle identity check.
You should fit your number plate with one way screws, but a strong adhesive will probably work OK. I believe modern plates are being made thinner and more brittle so that they will shatter (like a car window) if you try to remove them.