This will always return false since the absolute of any number is always equal or larger than zero: fabs(Sum([VolumeCT]) <0
You can try something like if(sum(VolumeCT),sum(VolumeCT),above(sum(VolumeCT))). That should display the previous weeks value, but that amount will not be counted in the subtotals. If you need that you'll have to fill in the missing values in the script.
Which symbol is used seems to depend on the Supress Zero-Values setting. If that setting is enabled the Missing symbol is used for missing and null values. If it is disabled then the Null symbol is used.