TL;DR We have (1) poor alignment of light into the cavities and (2) ghost beams interfering with our resonant light from inside the vaccan.  This could be the cause of our scattering shelf.

We have been messing around on the transmission table, trying to determine where are 50 Hz scattering shelf is originating from. We have a pretty good candidate for the scatter source in the north path fringer, with what appears to be a ghost beam coming out of the north cavity window.  The photo below was taken when the cavities were unlocked and with half wave plates making the back table light p-polarized, generating these beams ghost beams which are always there and interfering with the main light.

We took off the vaccan's foam on the back, and looked in with the IR viewer.  I can count at least five spots of IR light, some bright and focused, some diffuse and clipped. 

I'm going to play around with cavity alignment and try to eliminate the ghost beams.