Fuel Theft occuring just when vehical stops for less than 30 seconds

Dear All,
Thanks for your support.
Yes you are right it was a problem with the way we plugged the device. We had not used the same source for Power for both the gps device and the fuel sensor.
However now we have another problem which is very frustrating. We are using Chinese FLS from a very big company which we have visited.
The problem is that we are getting fake thefts and fillings just when the vehical stops for a few seconds( 30seconds or so) in traffic. Most of the fuel thefts and fillings are around 7 to 10 liters and take place within just 30 to 40 sec. This seems to be not possible as when you see the graphs you can see the Vehical engine was on and had just stopped for 30 sec or so. What kind of setting can we apply to avoid this.
The problem is the the sensor by default filters the data for over 120 sec by default.
I am very confused on how to solve this problem .. please can someone help
thanks