I talked with Isaac last night and evidently when the user is viewing a "cached result" from Bing there is no referrer variable passed as it's not "BingBot", but a user that's requesting data through a Bing server.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.