If it were an older car I'd think it was a bad heater core but it'd be unusual in a new Volvo (yet nothing is impossible either). Have the cars cooling system pressure tested and be sure the heater core area is inspected as that might be the last place anyone would normally look for coolant with a newer car.

Other possibilities: 1)Alot of moisture in the car, like if people are entering with snow on their boots or wet hair from just getting out of a shower. 2) recirc mode vs fresh air (should be less if fresh air mode is used) but even still you should be seeing fog all the time.