Fig.: to be suspicious, to feel that something is wrong

The bag-lady showed up every day at the time of the armored-car delivery and the security guard began to smell a rat

Quando la guardia giurata notò che la stessa mendicante si faceva viva tutti i giorni all'arrivo del furgone blindato, cominciò a sentire puzza di marcio

Police suspect the involvement of insiders in many artful scores. The smell of a rat is even more pungent when masterpieces vanish after video and alarm systems have been turned off and before guards start their rounds

Per molti furti d'arte la polizia sospetta gli addetti ai lavori. La puzza di bruciato è particolarmente forte quando le opere d'arte spariscono dopo che gli allarmi sono stati disinseriti e prima che i guardiani inizino la ronda

As I entered the room they immediately stopped talking. I smelled a rat and I'm convinced that they were badmouthing me

Come sono entrato nella stanza, hanno smesso immediatamente di parlare, ed io mi sono detto: “Qui mi puzza, ma mi sa che stavano sparlando di me”

When households rats were much more common than today, they would sometimes die inside a wall or under the floor and soon the householders would literally smell a rat. The rats we smell today are usually figurative, but smelling them still means that something is rotten. We don't know whether this or any other everyday image is at the origin of the saying, but we know that it has been in use since at least the mid-16th century, as a ballad from 1533 includes the refrain: “For yf they smell a ratt”