Ok, i have a v70, i was towing my caravan, and after about 50 miles stopped for a break and about a litre of coolant appeared underneath the car, the car had not been overheating when being driven, i then topped up the coolant level and continued my journey, i had no further problems on route, after unhitching , i used the car for a week covering approx 600 miles with no more loss of coolant, i then towed my caravan back home, the car didnt overheat, when i arrived home and unhitched my caravan the coolant had leaked again, it seems ok when being driven and pressurised but discharges coolant when stopped.

 

Pressurize the cooling system on cold engine, leave overnight and ck for leak in the AM.

Appears you have a leak that only shows up when the system is stressed, like when towing. Even though the temp gauge isn't in the red, I'd be willing to bet it's running hotter (hence higher pressure) when towing.