Methods to prevent oil theft from well site

Oges clients is facing the problem of oil theft from a well site. They believe that well produces more but reported oil volume is less.
All wells produce from 25bbl/day to 300 bbl/day. Oil gets collected in a tank on the well site. The gas gets released from a tank. Then a tanker comes and collect the oil from a tank and goes to a nearby processing plant.
As of today, they have put meters just after Xmas tree. These meters transfer values over the internet to their head office. They still feel that there is leakage somewhere.

What additional or better equipment and processes they can setup? Please share some examples and case studies.

You can also choose to provide your answer here itself which I will post on Oges under your name.

Looking forward to your reply!

Thanks in Advance!