。
She must have gone out.
Literal
She [topic-は] went-out [must be-にちがいない].
~に違いない expresses strong certainty — the speaker is quite confident she went out, based on evidence or reasoning. It's more definitive than だろう (probably) but not as absolute as a flat assertion. Often translates as 'must be' or 'no doubt.' The plain past 出かけた before に違いない places the certainty about a completed past action.