[Porsche 911]

Had a standard valve adjustment done about a month ago, including a complete oil drainage & fill-up of 11 quarts of 20w-50. At the time, oil level showed slightly overfilled at about 2mm above the upper hash on dipstick.

25 days and 730 miles later, oil level only shows 30% up from lower hash mark. I know a "healthy" engine eats up about a quart every 1000 miles, so I'm pretty certain this amount drop isn't normal.

I constantly peak beneath the engine when the car is parked to check for any signs of oil leak, but have not noticed any puddles or even drops of oil the last couple of months. I'm pretty certain I'm taking measurements correctly - after the engine temp is warmed, car idling on a level surface - so I don't believe that's the issue.

Any ideas for the rapid drop in oil level?

TIA