Are you able to look at the car when it is parked ? If there is any oil on the ground then it is easier to trace by looking up from the spot , Also look at the engine bay and see if you can see any obvious leaks from the head , the oil filter , or any other oil stained area , Be patient , use a flashlight [ even in daylight ] and take your time to inspect everything that you can see from the top to the bottom , You might want to take the car to a toyota dealer to have it inspected ,LEXUS DEALERS WILL JUST CHARGE YOU MORE $ Take a afriend who knows something about cars . carfax it ? get all the previous maintenance records , If none don't buy , There are a number of perfectly good used lexus on the market .