Maybe this a bit of a long shot, but having been told of the sizable lumps of ice which have been known to form in aircraft fuel tanks over a number of flights, if the conditions are right (dissolved water freezing out, and large amounts of condensation from frequent descents through moisture laden air) could the imbalance have been caused by one of these melting? This might give an increase in fluid flow, although the fluid would be fuel with a significant water content from melted ice.

AIR790C: Considerations on Ice Formation in Aircraft Fuel Systems - SAE International

I'm struggling to see how ice could form or melt in one side only; however the way the aircraft was parked relative to the sun might explain that.

This is not the BA038 scenario, more a case of a build up from condensation over a number of flights in the outer part of the tanks from a succession of flights and then parking in conditions sufficiently cold that the ice build up did not melt until the flight in question.

Could be Mechta, but BA have been hot on carrying out fuel tank sump drains, and are done routinely. If water is present in the fuel there will be also be a message on the fuel maintenance page. :)

Yotty, The water detector is an inverted ultrasonic fuel probe which looks for the water/fuel interface, and is fitted at the inboard (lowest) end of the tank. If the water in the fuel didn't make it to the water detector, having frozen first, or was so much that the water detector was fully submerged (from what I remember, its only about 6 inches tall), then perhaps it wouldn't have shown up on the maintenance log?

I guess that is entirely possible Mechta!