Fuel data analysis requires a deeper look if false refills/drains are suspected.
The main reasons for incorrect records of fuel drains and/or fill-ups:
tracking device has power supply or ignition issues;
vehicle operates or drives on an uneven surface causing fuel splashing in the tank;
the vehicle is tilted;
vehicle is being transported on a trailer;
malfunctioning fuel sensor in a two-tank fuel system;
fuel flow while the ignition is off in a two-tank fuel system with CAN-bus data;
non-standard shape of the fuel tank on special vehicles.
More useful information on drains can be found here.