Doors and Floors Level 12 Walkthrough: Pick up the lighter on the ground near the bottom left of the door.
Look at each of the 5 symbols above the door. Split them into 4 equal pieces and you'll get a letter or number: ODB7C Press the semi-hidden keypad to the right of the door and enter: 03272 Tap the door while you hear the buzzing sound. If the sound stops, enter the code again.
Special thanks to Dave, Jeffrey, Alexandre, and Kristy :)