My week of SSI:
BDL, on the metal grated cover on top of the X-ray rollers, they have training manuals or some other kind of book that read something along the lines of "SSI," on the top cover.
So I thought, ok, it was one lane that had an idiot that just left a book out in the open, oh no, they were in every lane.
I wanted to photograph it, and/or grab it and see what happened, but it was too early.
....Then, at TPA, they leave all the X-Ray monitors on overnight and in plain view when the checkpoint's closed. So maybe it's only the elusive SSI during the checkpoint's normal operating business hours. All other times, it's just a silly x-ray screen.