For a moment, Loki considered coming up with a story, some lie or other that would make him sound especially clever or sneaky. But really, the truth was good enough, and he found he wanted to tell it for once.

Quite possibly he just wanted to brag. Almost certainly, since there was no way Thor of all people would be impressed. "Since shortly after you left me in the Dark World. While you were on Midgard dealing with Malekith, I returned to Asgard. Disguised, of course. When you returned to give up the throne, I was already seated on it." Just to rub a little more salt in, because if he couldn't get praise he'd take anger, Loki smiled at Thor, bright and cheerful. "It's been years, brother."