I think that if you see a signal in the demodulated PDH error point with the cavity unlocked, that this must be RF AM on the light.

What mechanism would produce this much AM? It can't be made through common path modulation of the carrier and sidebands. It must be an etalon formed somewhere between the EOM and the input mirror of the suspended cavity. Could be windows / viewports; this can be tested with the ND filter insertion technique we discussed on Wednesday.