Theoretically speaking, as Orb was his tenant, Loki was supposed to knock on the door when he went to visit her.
But over the years – year? How the hell long had it been that she'd been living there, anyway? – since she'd moved in, Loki had come to know a few salient facts about his tenant; among other things, that – until she was going to bed, at least – she never really bothered to lock the door to her flat from inside the building unless there was a show on downstairs and she wanted to make particularly certain that no one would try to come in.
Meaning, in a roundabout-logic sort of way, that she hadn't shut him out, and therefore – even more roundabout – she'd invited him in.
All of which was to say: when Loki went to see her, nearly tearing his hair out with nerves, he didn't actually bother to knock. He just let himself in, aiming for her bedroom on the grounds that that was where she had to be when she wasn't in sight just inside the door, and so he heard her humming, and stopped short at her bedroom door itself.
"Oh, now that's lovely," he murmured, when she paused, not wanting to interrupt her.