。
She was just about to leave the house.
Literal
She [topic-は] precisely house [from-を] leave [about to-ようとしていた].
を marks the departure point with verbs of motion — a non-obvious use of the particle that English speakers often miss, since を usually marks a direct object. With 出る ('exit'), 降りる ('get off'), 卒業する ('graduate from'), the place being left appears with を rather than から. The construction まさに~ようとしていた pictures the exact moment just before leaving — frozen at the threshold.