The only Mk2 that this happened to was the amby. I would walk past and it would look like somebody was having a snogging match in there

. On closer inspection, there was a 1p sized hole in the roof, probably from something being removed, directly above one of the flouro lights. The light fitting was catching the water, then it would steam off when the roof heated up. Check you haven't got a slight leak somewhere it's not seen.