Hazy recall of the fab 767 but I seem to remember basic thinking ; "Low Fuel" in any tank required placing all pumps on & X-feed open. In tank to engine, fuel imbalance was another drill & a sometimes the result of the low fuel drill. There was an imbalance limitation too. Get down to that limit with Xfeed open suggested you were feeding the leak so the Xfeed was closed. You now had a possible leak, identified & out of balance & with refined drills after the Airbus (I think) incident suggesting the Xfeed should be closed. Sim Tripper-uppers loved that scenario ,
The FAA mandated a new EICAS message in response to the Airbus Azores' glider, basically to indicated a major fuel leak. I wasn't directly involved and don't know details, but basically it compares fuel used according to the engine fuel flow sensors with the fuel remaining based on the FQIS. If there is a large discrepancy (my memory says 7% but I wouldn't swear to that) it pops up an EICAS message (sorry, I don't remember the text).