1999 Lexus LS. Used engine installed 200 mi from home in son’s car. Used engine allegedly 100k on it. Oil loss at the rate of about 1 qt per hour. Got the car home, kept topped up. Strangely, no oil drops on pavement, no blue smoke out the tailpipe. There is some smoke coming out from under the hood. There is visible leaking from the valve cover gaskets. Could a car leak that much oil and have it all burn off on the exhaust manifold or something so that none gets onto the ground???
Go out and look under the car in the morning. Sometimes it takes awhile for the oil to make it down the Plinko board that is the underside.
Where under the hood is the smoke coming from?
A leak that large would make me worry about the rear main seal.
The oil might be leaking onto the exhaust manifold or tail pipe. Open the hood and see if there is oil on the exhaust manifold. If you can’t see it, the leak could be lower and you would have to get under the car to see it.
Again: The leak can be seen at the valve cover gaskets.
Again: There are no drops under the car.
Origin of smoke cannot be seen due to cramped
engine compartment. Blows around.
Again: could that much oil be burned off so that there is no drip on the ground?
Realize that there are seals such as crank and cam seals that have the ability to leak large amounts of oil only when the shafts that go through them are spinning and/or when the crankcase is pressurized as in when the engine is running.
My wild guess would be a crankshaft seal. But it’s only a guess. One thing I’d suggest is getting a UV sensitive dye with a blacklight (you can buy the kit). Follow the directions in adding the dye to the oil and the path the oil is taking will light up under the blacklight like a luminescent deep-water fish.
an example: Subaru Outback has a big “diaper pad” on the engine undercover, capable of soaking multiple quarts of oil or coolant before the first drop would reach the ground
get a flashlight, small mirror, raise your car if needed and get your answer fast 
did used motor have warranty from junkyard? 1qt/1hr use?
So, if it were going past the rings, as per my mechanics theory, there would be at least some blue smoke out the tail pipe, right?
Yes, it will drip on the exhaust manifold and burn. It usually smoke pretty bad under the hood and smell of oil burning in the cabin.
How do the spark plugs look?
A leak can be seen at the valve cover gaskets. It’s very possibly leaking in more than one place. A quart an hour out of the valve cover would be a very significant leak.