。
She might have left her car key in the room.
Literal
She [topic-は] car [of-の] key [object-を] room [in-に] left-behind [explanatory-の] [might-かもしれない].
のかもしれない combines the explanatory の with かもしれない (might), creating a speculative explanation: 'it might be the case that she left the key.' 置き忘れた describes the past action of leaving the key behind. 鍵 (かぎ) is the standard word for key. The speaker is hypothesizing about what happened — a natural use of かもしれない for uncertain past events.