Here is a link with cabin filter information. If your's does not have one a retrofit kit is available from Volvo. Easy job. You need to relocate the temp. sensor in the filter holder and the Volvo kit includes it and instructions.

http://volvospeed.com/volvo_repairs_how_tos/filter_replacements/pollen_cabin_filter_replacement.html

I had a similar slow loss or fluid and tried all the easy and less expensive suspects first. In the end it was the radiator. That was at 110k miles. Hope it's not because they are expensive.