Urban Quandary

You are walking up the stairs to your apartment. On the second floor, someone has left their keys in the door.

Facts:

• In the cold, the front door sometimes does not close fully. It opens with a push.
  
• "There have been burglars."
  
• The occupant of this apartment may or may not be Australian.
• The light two floors up has burnt out.
• There are no one-bedrooms, as far as you know, and therefore no one doesn't have a roommate.
  

Do you:
(a) knock on the door?
(b) open the door and call out?
(c) open the door and leave the keys inside the apartment, effectively locking the person out but also hopefully preventing them from being robbed? (They have a roommate, though, remember. And! the keys are in the doorknob, which locks with a button on the inside (if it's like your knob), so presumably they were on their way in when they left the keys in the lock? Yet no one answers your knocks.)
(d) leave the keys where they are and hope for the best?
(e) some of the above?