[tlb]

Indeed, but that system will report nothing unusual when there's a leak directly from the tank, or in the pipe between tank and flow sensor (which I think is usually near the engine, downstream of a fuel pump) or if the tank wasn't filled to spec.

Any sensor has a failure rate. If the probability that a sensor has failed isn't dramatically lower than the probability of a leak, then the pilots will do just what they did in the incident and assume a bad fuel sensor reading rather than a leak.

Another failure mode of fuel tank sensors is short-circuiting and blowing up the plane: https://en.wikipedia.org/wiki/TWA_Flight_800