What is "Tell Tell Hole"? Where does it exit?

in Wartsila Engines the tell tale hole is located in the liner body to be a sign of water leak from the water jacket around the liner 

Also there is a hole in the water pump for the same reason

First correct you term, it is Telltale hole.

As per API, telltale hole in Reinforcement pad shall be located substantially on the horizontal center line.