As you have watched the cut scene at the main gate of the quarantine section, you will find Elrath in his unlocked house and can chat with him. He will tell you that his key was stolen - is he really innocent? I advise NOT to take the key SE of his house before you speak to him for the very first time, if you haven't got lockpick level 2 - he will compliment you out of his house and it's locked now.