My paternal grandmother would sometimes threaten to put salt on my tail. I was never quite sure whether to take this literally or whether she was using an unfathomable metaphor. I was aware of the illustration of the boy putting salt on the tail of a live chicken on boxes of Cerebos salt but that did not seem to help. The phrase appears to refer to apprehending a criminal (Brewers - “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.”— Sir W. Scott, Redgauntlet, 1824, chap. xi) but begins with putting salt on an animal you want to catch. Perhaps the idea is that if you are close enough to it to put salt on its tail then you are close enough to catch it.