Where the cops AR doesn't fire.  Report said he fired 1 round and the mag in there was empty (no jam).  So this reinforces the habit of always check your gear.  Which is their SOP before starting shift. 

The other question is why didn't he use his sling when took the rifle out of his car?  Can't just leave an AR on the ground.  I didn't read where he came from, so it's possible he had to remove it for X reason.  But good problem solving especially since the shooter was right there.  Easier to transition to pistol than to tap/rack, or he knew the sound of an empty AR.  Which is amazing considering the situation.

Very good muzzle awareness by the bodycam cop.

So what can we learn, check your gear.  I learned this in the last HDF carbine class.  I do it regularly with the pistol, but no the AR.  AR sits in the safe without a round in the chamber.