I think my mondeo did something similar.
The coolant is pressurised as temp increases. so no leak at rest.
Whilst driving - pressurised coolant leaks to the outside world through
a crack or split head gasket.
This crack could also allow water to go via a cylinder so be steamed off
as Simon says.
Hi Kevin,
Your head gasket has gone. As there is no water in the oil it will be
leaking between cylinders and you will see white smoke (steam) coming out
the exhaust. Steam is normal while the engine & exhaust system warm up and
evaporate any water residue, especially noticeable on those chilly
