Leaving a door unlocked with 〜っぱなし
Grammar Explanation: Leaving something as-is with 〜っぱなし

Blue tries to sneak into Masaki’s cottage, but the door is locked.
- ブルー:
- 「開けっぱなし…なわけないか。まあいいわ。」
- “There's no way it'd be left unlocked. Well, that's fine.”
Key Points
開
( けっぱなし means left open開
( けっぱなし describes a door left in the opened/unlocked state after someone opened it.The focus is on the unchanged state, not the act of opening itself.
なわけないか rejects the possibility
Blue briefly imagines the door might be left unlocked, then says there is no way that would be true.
か at the end softens it into a self-correction: “I guess not.”
The phrase is useful for careless states
〜っぱなし often appears with things left on, open, out, or unfinished.
In this scene it fits because she is hoping for an easy way into the cottage.