Silent, slow moving light that eventually took off very fast.

I was going to guard duty one night while I was in Viet Nam.  We were on the mountain overlooking Da Nang.  About a dozen of us were standing around talking, while waiting to go to our posts.  One person spotted a light and pointed it out to all of us.  The light was changing colors and was unlike any light we had ever seen.  It moved slowly and quietly until it was almost over us.  It hovered for about 5 to 10 minutes.  At the last, it took off, straight up, at a great velocity and was gone in a second or two.  We spent most nights on guard duty and were used to all of the lights of the night sky in Viet Nam, and this moved in a fashion that no known terrestrial vehicle could move.