FrenchIl y a anguille sous rocheThings aren't clear or something is being hidden.

The closest analog I know is "Something seems fishy" though the phrase literally translated is "There is an eel under rock". This has connotations of perfidy and cowardice because of the old verb "guiller" and the general appearance of an eel being snake-like as well as its tendency to immediately bolt when discovered under said rock since eels are quite literally stumbled upon under rocks where they hide during the day. Thus the origin of the expression is both literal and etymological and interestingly it influenced the name Guillaume as a kind of conjugation of the verb.