I had a similar problem on my 2003 V40 Auto. I was losing coolant and could not work out how or why. i went through it all from water pump to possible head gasket.
Replacing the expansion tank cap was the solution in my case. I purchased a 2nd hand expansion tank and cap off ebay and quickly fitted the cap with a view to fitting the tank at a later date, but the new cap did the trick.
As I understand it, as the coolant system got under pressure, the coolant rose in the expansion tank as it should, but was seeping out via the cap which was not creating a full seal. Incidentally, the cap looked fine.
I would by a 2nd hand one like this for 10 GBP, it is worth a shot
http://www.ebay.co.uk/itm/VOLVO-V40-...0AAOSwnK9ZSkTH