Losing oil

I have a 2005 S60 2.5T AWD with 143,000 miles and I’m losing about 1.5 quarts of oil every 1000 miles. I don’t see any oil dripping under the car, but there is minor oil residue on the splash pan. I recently had the timing belt replaced and the mechanic said the seals were fine and he didn’t note any obvious oil leaks. So, I’m wondering if anyone can suggest where my oil is going.