Throughout Rapture, the player will come across doors (and one safe) with 4-digit access Codes required to gain entrance. The codes are usually found nearby or shortly after the lock via audio diaries, written notes, and scribbled messages. Many of the locks in BioShock can be hacked or bought out, but it is almost never required. In BioShock 2, hacking combination locks is not an option. Two locks can also be found in Columbia during Burial at Sea - Episode 2.
It is possible to open keypad locks even if the player has not yet listened to the audio diaries containing the correlating codes.
The first door access code in the Medical Pavilion is a reference to System Shock's first keypad access combination, which is 451. This is a motif that appears in other games such as System Shock 2 (45100), BioShock 2 (1540), BioShock Infinite, and the Deus Ex franchise. The first combination in BioShock 2 in the Adonis Luxury Resort is 451 backwards (1540). The number was originally believed to be a reference to Ray Bradbury's book Fahrenheit 451, but was later revealed to be the entry code to LookingGlass Technologies (later: Looking Glass Studios) offices and just a coincidence.