There's really only 2 ways to lose oil, consumption and a leak.
Consumption of a significant amount of oil as you describe should be noticeable if you're paying attention. Consumption is going to be through the rings or through the valve stem seals, both of which would cause a decent amount smoke while driving or on start up.
A leak is going to leave an obvious sign that you should see but there's people that don't.
Here's a post from FB of a guy smoking an engine.
View attachment 211396
To me, you should have caught this and known you had a problem. Having a dipstick wouldn't have helped him but this is exactly what I'm talking about when I say most don't pay attention and wouldn't even check a dipstick. You miss this, you're not checking a dipstick