I own a volvo xc90 d5 (engine code D5244T).
I have a problem with cooling fluid, which is being lost somewhere. There is no noticeable difference in engine performance, engine temperature gauge never goes past the middle mark, but "coolant low" message pops up now and then (which is easily resolved by slightly unscrewing cap on a cooling water tank). For every 1.000 km of driving I have to add cca. 0.1 liters of fluid. This problem persists for around 8 months now.
At the first garage, they replaced water coolant tank and it's cap. But that didn't help. They checked hoses for leaks but didn't find anything, so I took my car to another garage. They were also unable to find leaks. They checked for presence of exhaust fumes in water coolant tank, but everything checked out fine.
Now they suggest I change cylinder head gasket, but can't guarantee it will solve the problem.

I'm open to hear solutions, or at least suggestions on how to proceed.
Thank you