You didn't specify how many miles are on your car, or it's maintainance history, or how long you've had it, or how much you drive.

There are could be a few drops left under the car every few feet everywhere it goes. Eventually it would add up to quarts.

I'd first look at the turbo oil return line "o" ring. Mine didn't seem to be leaking much in my driveway, either. From Bay 13:

http://www.volvospeed.com/Repair/turborepairline.htm

Then the main seal, etc.

-bill d cat