Bright white light moves in U-pattern, then fades out.

Sometime after dark, the fire department was dispatched on a report of two overdue fishermen.  As ranking officer, I responded with an area captain to the bridge on Highway 7 – the point the fishermen were last seen--to coordinate response by the department.  As we pulled up to the bridge, I spotted a large bright white light downstream from the bridge at a point I guessed was directly over the confluence of the North and Middle St. Vrain creeks.
As I watched it, the light moved back and forth in a U-pattern.  My immediate association was to an airplane dipping its wings.  After a few seconds it faded out.  I had the mental impression that this light was signaling the location of the fishermen, and letting me know they were okay.  I asked the fire captain if he had seen the light, and he said no.  I told him what I had seen and what I thought it meant.
It was too dark to search downstream, so I assigned a couple of people to stay at the bridge overnight in case the fishermen walked out.  The next morning I led a crew down an access road to the creeks.  We had only been walking a few minutes, when we met the fishermen walking out.  They were okay, and reported they had spent the night “where the two creeks come together.”  I asked if they had seen any unusual lights during the night, and they said no.
At the time, I was living just upstream from the bridge, and I had seen a similar sized flashing white light in the same area, but a little further north and stationary.  I am aware of at least one other area resident who had seen a similar light in this general location on at least one occasion.
I am presently a psychoanalyst in private practice.  Before I changed careers, I was chief of a large fire protection district in the western U.S., and had training in fire investigation.   Before that I was a naturalist with the National Park Service, and have long been a student and observer of nature.  I consider myself to be a curious, careful and scientifically oriented observer.