maybe a Park-N-Ride lot? i believe part of the lot in the Southpoint Mall near the theater is technically a Park-N-Ride. i would call someone to confirm overnight parking is OK, though. check out
www.gotriangle.org for contact info.
aside from that, i would bet a lot without its own security would be your best bet. The Woodcroct Shopping Center on the corner of Woodcroft Parkway and Hope Valley Rd may be a good bet, especially if you park out in the boonies near the Bandito's and Karate vans (it'll make sense once you see them).
Mike