I was also thinking this. I don’t think it was BOB behind Sarah’s face. Also that smile looks more like Laura’s to me. My thought was that Laura’s mother could be possessed by the Mother from episode 8 who spewed out all the evil after the experiment.
Lots of people have been saying that Sarah Palmer is the girl in episode 8 who swallowed the frog bug, so maybe the Mother spirit entered her through the frog?
Regarding Sarah Palmer being possessed by BOB:
I agree that the teeth shown do look like his, but I don’t think the image is enough for a positive identification. The case made in your podcast relies heavily upon Frank Silva being credited, but we see BOB in the orb being vomited from the Experiment in Andy’s vision sequence, so that could be the reason for the Silva credit, which then leaves open the possibility that the Cheshire grinning teeth behind Sarah’s face belong to someone else. Some have suggested they look not unlike *Laura’s* teeth from certain smiling scenes.
So could Sarah be inhabited by the vengeful spirit of Laura Palmer? It seems hard to swallow (but maybe not as hard as a frog-roach). After all, Laura was abused by truckers in FWWM, and some people have noted a bit of a resemblance between that trucker guy and an older Leo Johnson. That’s not to say he *is* Leo – I think Leo was eaten by spiders – but he would be a worthy target of Laura’s avenging wrath.
I’m not married to this theory or any other, but I felt it worthwhile to question the certainty of the BOB inhabitation hypothesis.