what the...?

Was quite suprised to see this the other day. I couldn't see it from the office, because most of my view is blocked by planes, trees and buildings (it is meant to be an observing office).

I was a bit worried because there was nothing on the forecast, the upper air trace, synoptic chart etc. Later there were a few specks on the sat image. This cumulus cloud was one in a line, probably quite a distance away out to sea. They built up and then died down again. Impressive size for nothing much going on!