Q: Oil is leaking in SUV, but source can't be found

My car has approx. 58,000 miles and the following missing-oil problem only started yesterday. When I had the oil changed, they told me that my SUV was four quarts low on oil. It normally holds six quarts, so they added Mobil 1 synthetic oil and filled it. Yesterday I had to pull over and check the oil level because the check engine light had come on. But nothing showed on the dipstick. I was close enough to an auto store to walk and purchased six quarts of oil. It took four quarts before anything showed on the dipstick. I’m trying to figure out where the oil is going. There are no oil drips on the floor of my garage and there is no smoke when I’m driving the vehicle. I even had a friend drive behind me to make sure. The coolant is a green color and the oil isn’t light brown or frothy. It is the color that it should be. I’m diligent in getting my oil changed every 6,000 miles using synthetic oil. I’ve done two oil changes in between the previous one and yesterday. No problems there. I removed a spark plug to see if it was covered in soot or oil, but it’s not. It looked just like it should. Can you give insight as to where the oil is going?