Visible crew/equipment: When the fusion reaction starts to destabilize and Harry shouts, "Shut it off!" to Otto, a boom mic is reflected on the glass doors behind Harry. (00:37:57)
Visible crew/equipment: On the way to the theater Peter intercepts a police officer chasing a couple of criminals. At the end of that scene one of the police cars has a tremendous wreck that swings the car sideways. There is a clear shot of the driver with a black helmet on.
Visible crew/equipment: In the very last scene of Mary Jane looking over the city in Peter's apartment, there is a reflection in the window where the camera lens and cameraman are partially visible.
Visible crew/equipment: After he watches Mary Jane exiting from the theatre with her boyfriend, Peter is running in the alley to put on the costume. On the wet ground you can see the traces of the tires belonging to the camera car used to shoot the scene.
Visible crew/equipment: Boom mic reflected on upper part of phone when Peter puts a 50 cent coin in.
Visible crew/equipment: When Peter rides his motorbike, in the wide shots he's replaced by an obvious stunt double who wears safety goggles.
Visible crew/equipment: Film crew and boom mic reflected on lower left glass of cafe when Dr. Ock approaches.
Visible crew/equipment: The camera and a boom mic are visible in the police car lights in the scene directly following the first accident with Doc Ock.
Answer: While it's never mentioned in the film, the most likely explanation is that Harry, as a rich kid, grew tired of "slumming it" and decided to move back into his mansion.
Harry and Peter's relationship quickly turned sour after Peter refused to discuss Spider-Man as Harry felt Peter knew more than he let on. Peter is Spider-Man's "personal photographer," and Harry only wants to avenge the death of his father. It's not unlikely to assume Harry left to live alone because he was angry Peter wouldn't help him find Spider-Man.