May be a stupid question(s) ... but are you certain that the car is on a competely level surface when you check the oil? Also, do you let the oil drain for a few minutes before you check the level? I agree - it must be going somewhere. When my oil return line started leaking there was never any oil under the car. How's the top of the engine look? Sometimes the gasket of the oil cap will leak, it costs only a couple of bucks, I recently replaced mine. How many miles? Which model? My oil capacity is "officially' 6.1 quarts, so it will read a little low unless I put in that extra 1/10 qt.

-Punxsutawney Phil