POLICE vehicles on Hongkong Island have acquired an extra sheen as a result of the automatic car wash opened at the Arsenal Street headquarters. As we hear it, the police cars have been queueing up for a brisk shampoo and rinse just to make sure they look bright when patrolling the mean streets. However, P.S. heard it took no account of the extra height of vehicles bearing a flashing light on the roof and wrenched one from its mountings. Since then only staff cars have been taken through the machine. But Francis Li, of the Police Public Relations office, insisted ''sensors have been installed that control the brushes; these things are designed to deal with all sorts of obstacles''. ''The cars with rooftop lights may not be as clean as the other ones - especially on the roof - but they can go through.''