ever forget something? On your car? It's bad, but when a cop puts a semiauto ar15 on the trunk, and another doesn't bother with the required vehicle inspection, and drives out into public... it's worse

http://seattletimes.nwsource.com/html/localnews/2015452756_coprifle29m.html has the full story, here is the essential info

According to two sources with knowledge of the incident, it began when one officer was inside the secure parking garage at the West Precinct and unloaded his equipment from a police cruiser. The officer placed the rifle on the trunk of a nearby patrol car, forgot about the rifle and walked off.

Another officer then went into the garage and drove away in the patrol vehicle without realizing the rifle was on the trunk lid, the sources said. (it's procedural requirement to do a walk around pre operation, make sure the lights and siren work)

The patrol car was parked a few blocks away from the West Precinct in front of the Roosevelt Hotel on Seventh Avenue around 9 p.m. Monday when at least two people spotted the rifle on the unattended vehicle.

A man who identified himself as Nick Gonzales confirmed he snapped a photo (this photo above) of the unsecured weapon.
Police said he then flagged down a couple of nearby bicycle officers.