Salt on His Tail (Lay). Catch or apprehend him. The phrase is
based on the direction given to small children to lay salt on a
bird's tail if they want to catch it. “His intelligence is so
good, that were you to come near him with soldiers or
constables, ... I shall answer for it you will never lay salt on
his tail.” — Source: Dictionary of Phrase and Fable, E. Cobham
Brewer, 1894 More on Salt on His Tail from Infoplease: