Why did Sarah Jane believe the Doctor was coming back for her? I know he was called back to his homeworld, but had he ever left her on Earth before and returned?
The Doctor had been recalled back to Gallifrey, a place he was unable to take Sarah-Jane so he dropped her back home (‘The Hand of Fear’). They never said goodbye and Sarah-Jane would have assumed the Doctor would come back for her once his business on Gallifrey was concluded. Though Sarah-Jane met various Doctors in ‘The Five Doctors’, her memories of this event where most likely wiped by the Time Lords to avoid possible time paradoxes. Similarly, Sarah-Jane encountered the 7th Doctor in the novel ‘Bullet Time’ but time manipulation occurring around Sarah-Jane at the time very well could have resulted in her memories changing once again.