wx balloon
Had an encounter with a wx-balloon some ten years ago. Same place as some of the other reporters; over Southern Germany, north of GVA. In the mid-thirties, missed it by a couple of meters. Bit of a scare actually. Reported it to ATC, but never heard anything about it.
So it happens occasionally. It was my only encounter in 25 yrs of flying. The big sky theory seems to work....