When you have gotten to the theatre, you will find yourself at a locked door. It has one of those multi-button locks which means you need to find the code as there is no way to manually do it. There are some audio diaries nearby that give some clues and the dead body nearby will give you some additional clues about finding an electrician as he has the password.
The quest objective does not correctly update for this quest so you will need to find the electrician yourself. The audio diaries and bodies will tell you that the electrician was in love with a robot that worked in the cloak or coat room. So where is the coat room.
The video guide below will show you exactly where to find it. You will need to head back to the lobby that has the main door you need to unlock. There are two big masks above the door to signal you are in the right area. The coat room can be found in a room to the left.
Once you have found the room, you will need to unlock some doors and inside the coat room you will find the electrician’s body. There will be a page in the electricians hand and this will contain the code for the door.