Police believe four teens were randomly targeting ­unlocked cars parked in a Rydalmere street when they opened the door of one vehicle and hit the jackpot.

It was an unmarked NSW Police vehicle and inside was a detective’s semiautomatic Glock handgun.

The Sunday Telegraph can reveal details of the urgent ­investigation under way to find the gun after the detective ­accidentally left it in the car overnight last month.

Four mates who went to the same school — three 16-year-olds and a 17-year-old — have been arrested in the past few weeks, but the gun’s whereabouts is still a mystery.

The teens’ alleged path of petty crime started on the night of October 9 in affluent northwest Sydney’s suburbia.

A black BMW 740i was pinched from Jezebel St at The Ponds. Inside the car was a Rural Fire Service uniform.

The next day police allege the stolen sedan pulled into a Rydalmere street, sometime after 6pm. During the night, the teens tried doors on dozens of cars parked in three different streets.

But all they managed to find was some loose change — enough to “buy a family meal from McDonalds” as one source put it.

BIKIE CHARGED AFTER GLOCK FOUND IN RAID

THE BABY GLOCK AT JOHN IBRAHIM’S GIRLFRIEND’S PLACE

That was until they unwittingly came across an unlocked and unmarked police vehicle. Earlier in the night an officer had driven home from work and left his gun, out of view, in the vehicle.

The State Crime Command officer learned his weapon had been stolen when he went out to his car about 5.30am the next day.

RELATED:

The detective, with more than 15 years’ experience in the force, is now at the centre of a Professional Standards Command investigation and could face criminal charges for not properly storing his firearm.

Within a week the four boys had been charged with various offences. But none have been charged with stealing the gun as it hasn’t been found.