Hello P-?
Many thanks for being in touch with the forum.
It is obvious by your description, that the car was actually - externally leaking - coolant, and being pessimistic, I think there would be little chance of anyone on here being able to diagnose the exact source of that leak without inspecting the car.
Moving forward:- you certainly did the right thing by returning the car to the supplying dealer, and OK, they couldn’t replicate the issue. It is possible, just possible, that they did in fact find the source of the leak and fixed it. This is probably an over reaction, but when you pick it up and check the coolant level before starting it, have a look at the surface of the coolant in the reservoir to see it there is any evidence of any slightly metallic looking deposits floating on the top of the coolant fluid. If there is, this might indicate that a leak sealer has been added. I would think not, since you stated the leak was external, but……
If it were mine, I would then replicate your journey and obverse what happens. Irrespective, of the outcome, I would then be booking the car in to a trusted local independent and getting the coolant system tested - and reported on, if found to be at fault.
Perhaps you could let us know how you get on P.
Kind regards,
Gareth.