verb/i.j‿a.vwa.ʁ‿ɑ̃.ɡij su ʁɔʃ/
for there to be something fishy and suspicious going on, for there to be something in the wind, in the offing
“il y a anguille sous roche” — I smell a rat; something is afoot.
Literally, “there is an eel hidden beneath a rock”. Compare also Latin latet anguis in herbā (whence English snake in the grass), as well as Dutch addertje onder het gras.