White and Red Flashing Light in Prosser Washington Horse Heaven Hills

I was driving to a convenience store in Prosser, Washington, taking the I-82 between Grandview and Prosser.  It was overcast, raining and thundering.  Noticing a bright white light on the side of Horse Heaven Hills, not really paying much attention but looking over at it from time to time.  
As I pulled into the Loves, I noticed it was now blinking white and red brightly, then dulling turning off and on. I got out of my car and moved out to the side of the building and watched it.  I was looking South - South East, roughly bearing 175 Degrees.  It was moving back and forth East to West.  I watched it for a minute before going inside to get a few things.  
Coming back outside and still seeing it do its thing, I decided to try and get to where it was.  Driving through town taking SW Bennett Ave.  To Highway 22, I had lost sight of it and I could no longer see it due to maybe the top of the hill.  Not having a place I felt comfortable pulling over I simply continued on home.
We spoke via telephone with the witness, and he sounded to us to be both sincere, and sober-minded.  PD