The other day, outside the Mairie of the 3rd arrondissement in Paris I spotted a long queue of bikes. At first I thought they were signing up for some kind of road race but as I got closer I could see that this was the latest campaign aimed against bike-theft in Paris. For the next few days you can come here and get your bike 'tattooed' for free. It then forms part of a national register making it easier to trace if stolen.
Bike theft is rife in Paris virtually all the cyclists I know have had a bike stolen at one time or another so I hope this helps.