。
She had left the front door unlocked.
Literal
She [topic-は] entryway [of-の] key [object-を] not-lock as-is [in-に] was-doing.
Two patterns combine. 鍵をかける is the standard idiom for locking — literally 'to apply the key,' using the same verb かける that pairs with many abstract objects (電話をかける, 'make a phone call'; 迷惑をかける, 'cause trouble'). The negative ~ないまま ('without doing X / in the state of not having done X') describes the persisting condition — the door was sitting unlocked. ~ままにする ('leave it that way') closes with ~ていた, marking ongoing/habitual past — she had been making a practice of not locking up. Japanese homes have historically operated under low-crime norms, and older generations especially could be famously casual about locks; this kind of habit features in countless mystery novels as the convenient setup.