Hey Everyone thanks for the replies!
The Frame is grounded at two points being opposing heat-sinks.
I can include some photos if necessary?
I have a multimeter and have been looking for short wires but nothing is coming up, does anyone have any tests that could improve my chances of finding it or a link to a guide somewhere?
I'm not 100% on the principals of electronics but if the unit is grounding to the frame like suggested how could it be that the screws that contact the "metal"of the frame carry current?