The expression il y a anguille sous roche literally translates as there is (an) eel under (the) rock, and can be used anytime you feel that there is something fishy going on, that someone is being dishonest or hiding the truth on purpose. Call it anti-eel prejudice or eel profiling, but with their snake shape and slimy appearance, these sea creatures scream bad news. They hide under rocks to shun the light, and burst forth only to snatch unsuspecting preys. Anyone poking around a reef would feel apprehensive about encountering an eel. It is this suspicion that something lurks around the corner that the expression describes.
The expression Il y a anguille sous roche is attested by Rabelais in Pantagruel as early as 1532, but it is certainly much older. Up until the middle of the 18th century, 1/3 of the French territory was made up of marshes - most of it was later cleaned up and replaced with agricultural fields. Those soggy areas were a perfect habitat for eels, and it was quite common to dislodge one by lifting a stone under water. If you could catch it, you could even turn it into a tasty pâté.
Snakes have always been regarded as a symbol of perfidy and treachery - remember Kaa in the Jungle Book? Even worse than snakes, eels were seen as wet snakes and so became the ultimate symbol of deceit.
J'ai refusé son offre parce que je sentais qu'il y avait anguille sous roche.
I refused his offer because I felt like something fishy was going on.
Un ordinateur qui ne coûte que 50$? Il y a anguille sous roche...
A computer that costs only $50? Something isn't right here...