Well, one thing for sure, the missing oil has to be going somewhere!
I assume it's a petrol engine, so it could get through a fair bit of oil without leaving a visible blue trail. Our old Cooper "S" Minis could "use" a pint of oil every 50 to 100 miles on rallies without leaks or blue smoke.
Once again, it's time to use LOGIC.
There are only so many ways for oil in the sump to escape!
Your assignment (should you care to accept it) is to find out where.
Up past the pistons? If engine running fine, probably not. Check the 4 spark-plugs first. If it is burning oil they (one or more) would be carboned up all black and horrible , and not that nice clean dry gray look the Haynes manual shows for OK plugs.
Leaking out? Maybe, but more than 6 drips looks like an oil change gone wrong. You would surely notice .... have you gone under to look along the bottom of the bodyshell?
Gasket failure - oil into water? You'll spot this in the coolant. It will look like a mess of bad mayonnaise.
We all want to know what you have found out when you find it!
Meantime what I would do is to DRAIN the sump out. Catch the oil for re-use unless it is needing changed anyway. Now find out how much oil it takes for a "oil-change-without-filter" oil change. Measure that much oil and tip it in the hole on top of the engine. By the way, make sure the drain plug is replaced first. Go for a cup of tea or a beer. Ten minutes later pull the dipstick out and check where the oil level is at, now that YOU KNOW it is right.
You see, maybe the dip-stick is telling fibs for some reason! I have had it often enough. I think that's why they are called "dip-sticks" ....
CharlesY