。
She might not realize the danger.
Literal
She [topic-は] danger [to-に] not-noticing [might-かもしれない].
気付いていない is built from 気付く ('to notice, become aware of') in its negative ~ていない form — 'is not in the state of having noticed,' i.e. 'hasn't noticed.' The に on 危険 marks what one might be aware of; 気付く almost always takes に for its target. Layered on top is ~かもしれない ('might, may, could be') — the standard hedge for uncertainty about a fact. The combination 気付いていないかもしれない ('might not have noticed') is a useful way to suggest someone hasn't picked up on something without claiming certainty either way.