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
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XC60 / Odyssey