This door is to an empty room that the guards use to beat prisoners. You can not open it but you can get inside. Enter the prison and get a visitors pass, wait for the guard to walk down the hall and he'll get a prisoner out of a cell. They'll return and open this door, they'll enter and two guards will attack the prisoner. After a couple of blows the guard takes the prisoner back to his cell.

Don't get stuck inside the room or you'll have to wait until they decide to beat the prisoner again.