First of all, she does appear to be talking into a cell phone. Secondly, the woman is not in the frame by accident. She isn't just some homeless bag lady talking to herself who happened to wander onto the movie set. That wouldn't happen. She's there by design. Whose design?
I vote for a clever bit of film editing by some bored prankster who had the tedious job of transfering celluloid films to DVD. An expert could tease this out, to see if the figure is consistent with the rest of the set.
Or look for another copy of this film footage and compare for the figure.