It is the low side of the serial data line. I have never looked to see if the data line is differential or not; from your description it may simply be 0v reference.
Reason it may be there is the older batts had these 2 data lines separate wires that did not go thru the cell pcb; thus it HAD to be there. New batts like yours and some of mine have all 6 wires going down to the cell pcb and then those 2 on from there.
Ohm it to the cell balance 0v pin and see if it is the same or not if you wish; doesn't matter to me.
