。
She'll be right back.
Literal
[topic-は] before-long return-comes.
戻ってきます is the polite ~ます form of 戻ってくる ('come back') — a confident assertion rather than a speculation. The shift between でしょう and plain ~ます in this kind of context shifts the speaker's stance: でしょう leaves room for slight uncertainty, plain ~ます presents the return as a settled fact. The whole sentence is the kind of line you'd hear at a service desk or office reception, telling a visitor that someone has stepped away briefly.