。
She might not know we're here.
Literal
She [topic-は] we [subject-が] here [in-に] are [nominalizer-こと-を] know-not might.
Two layers of grammar produce the hedging. (1) The nominalizer こと turns 私たちがここにいる ('we are here') into a noun-equivalent that can serve as the object of 知る ('know'). (2) ~かもしれない ('might / maybe') hedges the main predicate to express uncertainty. The negative 知らない ('doesn't know') under かもしれない gives 'might not know.' 知る vs 知っている is worth noting: the bare negative 知らない works fine ('doesn't know'), unlike the affirmative which requires 知っている.