if you had that big a leak, would you not be aware of it?, also the supply tank would be reducing at a steady rate (as the engine burns it) that should be familiar to the pilot?
When everything is normal the supply tanks stay full until the main tank is depleted. As the main tank may have been indicating 75kg, one might easily assume the abnormal condition is diagnosed as a supply tank level falling rather than the other supply tank level remaining where it should be.
all that aside, if I belives I had a major fuel leak and was going to be on one engine anytime soon, I would chicken out pretty fast, call mayday and put it down ASAP.
The fuel system is compartmentalised to cope with this situation. It might go back to Sid's question, with an apparent 46kg of usuable fuel and minutes from base, why would you land a serviceable aircraft?
If, from the indications you have in the cockpit, you diagnose a fuel system fault, i.e. a leaking supply tank, you wouldn't be too worried about an ear-roasting for landing below minimum fuel, presumably?