HGF or not ?

Just filled expansion tank for 2nd time in a week over 150 miles .

First time the cause was obvious ,hissing cap and evidence of coolant loss from cap .

Replaced cap ,topped up and bled ,all seemed good .

Someone else just drove the car and noticed water while reversing ,from gearbox area .On inspection the tank was empty and took approx 1 litre to refill ,but no sign of another faulty cap .

The car starts and idles fine ,no steam from exhaust ,no mayo on oil cap or in coolant tank and temp guage has always been just under half .

Any ideas ?

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