I have seen police bikes, ambulance bikes, and Secret Service bikes. Never once though have I seen them locked up. They have always had a rider on them or have been within sight distance of the operators. Reader Ben however has seen a locked police/security whip. His text and picture follow:
I took this picture back in 2007, on Michigan State University's campus in East Lansing Michigan. The bike is one of the ones used by MSU's police, and has the decals to show for it. If you look closely you will notice that the bike is locked down using some of the officer's own handcuffs, which I thought was quite humorous.