Hummm, stange this.
Last week I put a couple bottles of food colouring dye in, to try and trace the leak. It did 3 return trips to work (so ~150 miles), without using a drop of coolant. Note the trips were free flowing, still holidays so lighter on traffic.
Spent an hour on Saturday with car on ramps trying to locate leak, without luck.
Drive it to work on Monday and we had grid lock, due to traffic issues. 2 lanes out on M3 and local roads all blocked. So ended up parking for ~30 mins and doing work on laptop before carrying on, but still very heavy traffic. Not a great run back either!
Check coolant level at home and half the header tank was gone.
Going to check at lunch for signs of colouring dye, but still no pools under the car or anything. Inspections following all coolant hoses showed no sign of previous leaks.
it appears excessive idling is causing it to drink coolant.