My guess is "C non-Schengen". xNS is usually used for a bus gate. Although there are some bus gates in C they don't usually take passengers by bus to C gates.

If you are talking about this morning's arrival from DFW, according to flightradar24 the aircraft which operated the route (D-AIKI) left for IKA this afternoon from gate C14. Since they usually dock an aircraft based on its next destination, I wonder if LH439 actually arrived at C14. On the other hand 8 hours (from 0910 to 1725) is a long time to park an aircraft at a gate in FRA.