It seems obvious that is has something to do with the bottles on the shelves behind the safe. I don't know if it's due to my brightness configuration (perhaps too dark), but I wasn't able to see anything over there, or to find any item in the bottles... So where is the combination ?
The first key code combination in LGS games, and since their demise companies who follow on in their spirit, is 451 or a variant thereof (e.g. 45100). This is a reference to Fahrenheit 451.
This means you can guess the combination quickly without even looking!
This tradition started in System Shock 1 and is followed by System Shock 2, Deus Ex, BioShock and now Dishonored :)