There are a few MRO (maintenance, repair, overhaul) facilities located at SAT:
http://www.sanantonio.gov/aviation/a...nce+and+Repair

I suppose NW could have a contract with one or more of them, perhaps as a result of the pending/ongoing mechanics' strike from the last few years? Pure speculation on my part, though.

I do know for certain (see my last post) that they're not doing the 757 winglet retrofits at SAT, though.