As my wife and I were driving westbound on the 91 freeway, about 2-3 miles east of the 15 freeway, we both noticed what appeared to be a VERY LARGE airplane or helicopter. Right away we thought military craft, no biggie.

As we watched the craft we noticed it wasn't one big craft but a regular sized helicopter following a ball of light.  It gave the illusion of one big craft.  They were travelling south along the east side of the 15 freeway.  The helicopter was following about (best guess here) 80-100 feet behind the ball of light.  Then either the ball of light slowed down or the helicopter sped up and was about 30-40 feet behind the light.  
Then as the helicopter got closer the light just went out.  The helicopter then just stopped and hover for awhile before circling the area before leaving to the south.  
I went to Youtube to see if any one else saw and maybe recorded the event.  There was nothing, however there's a video from a few years back filmed in Anaheim (about 5-6 miles west of our sighting) that was very similar to what we witnessed.  
I'm actually shocked having this happen at 8:30pm over a busy California freeways and there are no other reports or even videos/photos.