A while back there was a thread here about the possibility of noisy switch mode power supplies (SMPS) spontaneously generating valid X-10 PLC codes. Bruce Robin sent a noisy SMPS (in a charger) that was blocking X-10 activity to Jeff Volp who ran some tests. AFAIK those tests failed to show any spontaneously generated X-10 codes.
There was a recent thread on Smarthome's Insteon forum that, in part, discussed phantom PLC codes being logged that went away when a CFL (with an apparently noisy SMPS) was removed from the mix. Further investigation found that the source of the apparently spontaneous codes was actually an ACT CR134 coupler/repeater. When it was replaced with a plug-in passive coupler, the phantom codes disappeared. Another factor was that the phantom codes only appeared after a valid Status Request was transmitted.