It was an optical illusion. I've seen similar in flight, more than once, and each time was able to see how it was being formed after a bit of observation and knowledge of physics.
I would LOVE to see something real, but I am quite sure it won't happen as I no longer believe there is anything there. I have to say that the first time it got my attention, until I figured it out. Subsequent times I was able to see what was happening much more quickly, but there always that first moment that gets the heart rate up!