Strange coolant loss.

[FLYING BANANA]

Today I attended the MGOC meet in Hayle Cornwall. Met up with two other cars and we convoyed the 61 miles to the event. Stopped outside the clubhouse and Banana dumped half of her coolant out of the overflow pipe on the catch tank. Lots of steam coming out of the near side edge of the bonnet.

Parked up and looked for the cause. Nothing to be seen on top but underneath, again on the near side, there was water dripping down via the undertray.

Left her to cool down for about an hour. Took 3 litres to re-fill. Started her up but left the radiator cap off. No bubbles, no oil in there. Ran the OBD and the temperature gradually crept up to 104 and the fan cut in. Still no loss. Turned her off.

Before leaving to come home I checked the level and it had dropped, not much, but down a little.
Drove home and she pulled like the proverbial train. Got home with no steam or coolant loss.

On the way down, and home I ran the OBD, and the temperature never went over 88.

Could it be the PRT?

Any ideas?