I don't think that there is any "master", or "overide" code. The actual code was probably set by the former occupents.
Try some dumb codes like 12345, 123456, 1234567, 111111, 22222, ...
If there is a connection to some central station, try communicating with them, tell them that the former occupents left without giving you the required code. Check for some phone number on the central box.
If you can't find a code like that, you probably will have to remove that alarm system and get your own.
Electro, March 2013