Another thought:

Check the oil level after the car has rested overnight. With so little mileage and conventional motor oil, the oil passages in the head may be blocked somewhat, and the oil will not easily flow back to the sump. This was the case on the poorly maintained car I bought, I also thought it was drinking oil on a 1/2 hour run. The next morning, the sump was over full. I cleaned the engine using synthetic + 1 quart of ATF for a tank full off gasoline, then flushed and filled with synthetic.

Four quarts lost in a month, but no leaks or smoke... Yeah, I'm right there with you, Where the hell did it go? Also, odd that an oil level warning light didn't come on as I thought all the 6-cylinder models have that feature. It may be a one-off occurrence, stranger things have happened. May be that now it's using oil and you'll have to add periodically.

I suggest poking all around the engine bay, above and below, looking for leaks or anything else out of sorts. Then leave your own car at home and drive this S60 yourself for a while. Look for any unusual behavior and keep an eye on the oil level.

Or, if you're like me and the anxiety and worry really gets to you, take it do the dealer or a Volvo specialist and have them address your concerns. Your and your family's peace of mind is worth the cost.

Best of Luck,
Will
--
XC60 / Odyssey

Besides the mysterious loss of oil, another big question is why didn’t you get a low oil warning.

With a flashlight, look around the turbo concentrating on the oil supply line and the oil drain line back to the sump. Any oil leakage from there will go directly to ground and probably only while driving.

Next, get a compression test. Let's hope it is not a head gasket leak.