This has happened to me a few times. For my 2007 V50 it was the drains that channel water away under the moon/sun roof. I run a stiff wire probe (10 guage copper) down into the little drain holes to loosen up any smutz and then pour bleach water into the trough slowly and let it drain away through the holes. I have noticed that the problen could be worse when I park with the front higher than the back. It usually happens during a heavy rain. To get the water up I use a wet/dry shop vac. I put the front seats forward and there is a seam in the carpet where I can get to the water better, but I also vacuum water through the carpet itself. I have also had saturation in the front floor wells and extend to the back seat. Check your front floor too. There is a lot of insulation under the carpet that could hide the waters presence.
Good luck,
John