Smell A Rat Meaning

(idiomatic) To sense something suspicious.

Example: 1663, John Dryden, The Wild Gallant: A Comedy, act 4, scene 1:
  Oh, are you thereabouts, sir? then I smell a rat, i’faith; but I'll say nothing.
1874, Anthony Trollope, Phineas Redux, ch. 40:
  Mr. Gresham, when he heard this, thought that he began to smell a rat, and was determined to be on his guard.