Bike security

Cycling over Blackfriars Bridge this morning I saw lots of police. At first I thought it must be an accident but it turned out the police were marking and registering bikes so if it's stolen, there's a chance to get it back. They did about 100 during rush hour this morning.

That's my bike getting marked. But they didn't ask for any proof that it's actually my bike. So any bike thief could have had a stolen bike registered to them!!