So he had no control over it? It wasn't a portal or a spell that he chose to cast.

[Penny clarifies. Because while he's not exactly eager to leave if he can return to the same moment in time he left from, he does want to be able to leave eventually. There are a lot of questions about aging and growing his magic that he still hasn't asked. And if the departure comes with a powerful memory charm, that's another hurdle to overcome, but hopefully he can train up his magic in the interim.]

If they're back in another world, how do you know they forget?