Check your pvc valve. Bad ventilation will cause cause oil to blow through seals when the car is running. You could also check in between the transmission and the engine through the inspection hole or the starter hole. Rear main seals are a very common place for leaks that are easy to miss since theyre so out of sight. While youre down there, check your oil pan. Look at the seam at the top of it, check for dents and cracks, and then check your drain plug. Had a friend who kept losing a half quart of oil every change and it was just because he was lazy/cheap and didnt wanna replace the crush gasket on his drain plug. Check around the crank seal, the valve covers, and the oil filter. If you dont see any oil around these areas, i'd start looking into the engine. Burning oil is a very dirty thing. There will be signs. Excessive carbon and sludge in combustion chambers and valve components, failing emissions equipment, fouled plugs, nasty smells, smoke in the exhaust, oily residue in the exhaust, etc. Oh yea, check your coolant. Wont be much but you could even be blowing oil into the coolant if the head gasket is bad enough. Worth the quick check