Using Oil

Up until my last oil change at 105,000 miles, I didn't notice any oil usage. When draining my oil, it didn't look like it was quite 4 quarts. After the oil change, I ensured that I had the engine EXACTLY topped off to the upper mark on the dipstick. Around 600 miles later, I parked in the same location and have noticed that the oil level has dropped enough to notice. I am guessing that I will lose well over a quart between oil changes. Before I resign to the normal mechanical reasons (rings, cylinder wear, etc.), is there some other possible explanations? The loss doesn't seem to be gradual which has me looking for other explanations. I also had a Toyota mechanic go over the car and replace the other fluids and filters at 100,000 miles (coincidence I think ). Any ideas?