False positives aren't zero, yet they're also high enough to make people wonder why they were irradiated and didn't just go for the pat-down to being with. I'm not aware of any official number for AIT.

The TSA is working on ATR (Automatic Thread Recognition?), to take the human operator out of the process, yet they've tndicated way too many false-positives (more then even with the operator). Their goal is to get the number down to where they can get the operator out of the process, and pat-down only the error-location.