If my dipstick is at 1/2 it doesn't take much to get to 1.4 and then the oil light comes on. As
@SuperV8 says, fill it to full and monitor the mileage until the light comes on. Very to know how much oil you use when you say it was filled between 1/2 and 3/4.
From my own experience, I've had oil leaks identified and fixed due to:
- Leaking rocker cover - it's made of plastic and will fail over time
- Leaking oil filter housing/oil cooler - it's made of plastic and the the rubber seals fail over time
Basically, see if you can get a good look around the engine. Remove the undertray and see if there are any obvious signs of oil leaks. If you are happy there are no physical leaks, then it could be internal.
My car was doing DPF regens every 120 miles or so if doing local short routes and 160 miles on motorway trips. It's on 152,000 miles and now it's drppped to about 60-80 miles so it might be getting tired. What sort of mileage is your car on?