资料题：What did the police advise the car owners to do（　）。

The recent increase in car stealing has alarmed the police who are looking for what they now believe must be a well-organized gang of professional car thieves.Most of the thefts have taken place in the rich residential areas round South well Park, where last week alone 22 car thefts were reported to the local police.Of these only one has been found, abandoned in the Rochinghill Palace Road 20 miles away.The others have still not been found except for one which had been falsely reported missing.The owner later admitted that he had forgotten where he had parked it a few days before.An increasing aspect of the thefts is that nearly all the missing vehicles had been taken from locked garages.The owners now admit that they may have left the car doors unlocked, but only one owner was not certain whether he had locked the garage door.The police had therefore assumed that the gang find it easier to break into garages where vehicles are often left unlocked at night than into locked cars; parked on the roadside in daytime or at night.They advise car owners to lock up their cars even when they are kept in locked garages.