。
She just stepped out a moment ago.
Literal
She [topic-は] just-now stepped-out.
外出する (gaishutsu suru) is the Sino-Japanese 'go out, leave the house, be out' — written-leaning and slightly more formal than 出かける ('go out (briefly)'). The verb is built from 外 ('outside') + 出 ('exit, go out'), and like many する-compounds, it doubles as both noun (外出 'an outing') and verb (外出する 'to go out'). The polite past 外出しました fits well in customer-service or formal phone-call contexts: 'I'm sorry, she's just stepped out.' This is the kind of phrase you'd hear from someone fielding a call at home or a small office.