Noticed that i had lost a lot of coolant after a motorway journey!

had it checked out and there is no leaks from the system

any ideas???

In order of likelihood, Coolant bottle and cap, Thermo switch on radiator for fans, heater matrix or heater hose leaking at bulkhead, last but not least, and favorite if it only dumps coolant when really working the engine hard on long runs, head gasket or cracked head.

surely you would notice a leak from the coolant bottle or cap though in the garage!

surely you would notice a leak from the coolant bottle or cap though in the garage!

Not really, the cap is designed to vent at a certain pressure, and seldom occurs when idling. Two reasons for venting, over pressure due to combustion gas getting into the coolant (head gasket) or cap valve faulty and venting at too low a pressure.