This subject has always intrigued me because of the before-mentioned grey areas. My comments refer ONLY to B767 and any corrections are welcome.
My thinking is, 3 possible areas to be identified for the leak:
1. Downstream of the Engine Valve, in which case possible indications are higher fuel flow and greater Fuel Used (FMS) on the affected side. Engine Fuel Leak, which the Engine Fuel Leak Checklist should resolve by shutting down the engine.
2. Between Engine Valve and Spar Valve, in which case the only indication is affected Tank quantity reducing at a greater rate. Fuel Flow and Fuel Used (FMS) indications would be normal. Carrying out the Engine Fuel Leak Checklist should resolve by shutting down the engine (closes both the SparValve and Engine Valve).
3. Prior to the Spar Valve (and somewhere in the tank area); after shutting down the engine as per the Engine Fuel Leak Checklist, the indicated fuel quantity continues to reduce. Boeing does not address this situation and provides no guidance. Perhaps, the catch-all 'Commander's judgement' could be applied here, with the POSSIBILITY of re-starting the engine and consuming the remaining fuel (remembering that shutting the engine down did not resolve the initial problem).
An interesting incident was the 777-200ER departing LHR about 3 or 4 yrs ago streaming fuel out of the center tank via the Left Wheel Well. This was observed by the tower, other aircraft and left a 2 nm plume. It was caused by a 2.5 inch opening in the aft wall of the Center Tank at approx the 30,000 kg level. The opening was a Purge Vent and the cover plate had not been re-installed! Lucky there was no Rejected Take Off as the resulting brake temps would have ignited the fuel.
Input please.........