One morning at our gliding club, there was still ground fog when we had got all the kit out. As the met man predicted, the fog burned off by about 10.30am. Shortly after, half a dozen large white 'spheres' rolled, skipped and bounced their way across the grass. It was really quite surreal, and somewhat reminicent of the programme 'The Prisoner'.
https://www.youtube.com/watch?v=I6Ffr1U7KMY
When the car came back from investigating these 'spheres', it was full of 'Chinese lantern' balloons.
My guess is they were launched the night before and had landed in or just upwind of the airfield. They must have still been partially inflated in the morning, and when the sun came out they gained enough buoyancy to resume their journey.
Like Step Turn, we also find quite a number of mylar/melinex balloons on the airfield. One of our younger members found one which was a tall, blonde princess in a pink dress, which made him the butt of a considerable amount of leg-pulling!