This is not a common or well-known saying by any means, but it tickles my fancy.

月夜に釜を抜かれる
(Tsukiyo ni kama wo nukareru; “having your cook-pot stolen on a moonlit night”)

Definition:

Being caught completely off-guard. Catastrophic negligence, carelessness, or inattention.

Breakdown:

月夜 (tsukiyo) is written with the characters “moon” and “night” and means, as you’d expect, “moonlit night.” The particle に (ni), in this case, indicates a point in time – in English, “on.” 釜 (kama) is a traditional Japanese iron cook-pot, marked by を (wo), the object-marker particle. 抜かれる (nukareru) is the passive form of the verb 抜く (nuku), which generally means “to extract,” “to pull out,” but in this case specifically refers to theft.

Notes:

In the old days, a kama was a valuable and hard-to-replace household tool, so there would naturally be the need to make sure that no area bandits or random passersby walked off with yours. The best time for theft, of course, would be in the dark of the night. You might think, therefore, that a night well-lit by the moon would be safe – but if you make that assumption, relax your guard, and allow your kama to be stolen anyway… that’s the level of carelessness referred to in this kotowaza.

The verb may be given in uninflected form as nuku, or replaced with with 取られる (torareru), from 取る (toru), “to take.” It may also be elided entirely, in the shorthand form 月夜に釜 (Tsukiyo ni kama). My sources remark that replacing the verb with 掘られる (horareru), from 掘る (horu), “to dig,” is an error. Why someone would make that particular substitution is unclear, but there you have it.

Like many others, this saying comes from the traditional Iroha karuta sets; in this case, both the Edo and Kyoto versions.

Example sentence:

「バッグをベンチに置いて、ちょっとトイレに行ってきただけなのに、戻ってきたらバッグも席も取られてたんだ。月夜に釜を抜かれちゃったぁ！」

(“Baggu wo benchi ni oite, chotto toire ni ittekita dake na no ni, modottekitara baggu mo seki mo toraretetan da. Tsukiyo ni kama wo nukarechattaa!”)

[“I put my bag on the bench and just went quickly to the restroom, but when I came back both my bag and my seat had been taken! I completely let my guard down!”]