I've had this happen on a flight. We CRM'd it, called our MW people and asked for a confirmation of our suspicions and our intentions. We were LOSING fuel (totaliser), the qty in tank 2 was about 800 kg more than the full tank we started with, the O/J pumps in tank 2 indicated on with flow lines also green. The CWT had some 30 tonnes (from memory) and the CWT pumps were on. All main tank pumps were ON and tank 3 O/J pumps were armed (blue). No EICAS messages at all!
Our suspicions were that it might be a non-return valve problem on the output side of the O/J pump(s) in tank 2, allowing high pressure fuel to enter tank 2 via the still armed O/J pumps, and the full tank venting fuel overboard. We thought perhaps the pressure in the O/J output lines may cause the pumps to indicate ON (green).
We suggested a fix to MW. This was a temporary measure to hopefully stop any further fuel loss. We proposed switching off the CWT pumps and allowing the O/J pumps in tanks 2 and 3 to reduce quantities in tanks 2 & 3 by about 2 tonnes each (and balanced of course). Our next move was to restore normal fuel configuration, and see what happened.
They consulted, and got back to us to proceed. We did the above and all went normally afterwards.
As it says in the QRH foreword......you might have to improvise.
