Earlier threads discussed ECAM insufficiencies, including that the threshold for Fuel Imbalance annunciation was quite large -- sort of like the Oil Pressure light that's really a "you've lost your engine" light on many cars.
A slower leak may have given the crew enough time to carry out a remaining fuel calculation after running the checklist -- or should that have been done before the checklist
In any case, a properly designed ECAM would be keeping a running tally of remaining fuel against remaining distance and reserves
and notifying the crew when margins are eroding. This is the kind of thing that computers are very good at and should be doing.
And yes, a manual fuel check should be done at the usual hourly interval -- even though it'll only catch slow/medium leaks. In this case, it looks like you'd have to be running a fuel check every 15 minutes or so
