Ull have more luck follow them to an actual opening Ive gotten buttons,coins, trash u never know with an old stone Wall u could get Lucky & run across a cellar hole that would b cool. Ya just never know on walls lots of time to make them Long ago lots of work left lots of time to lose things
I've often wondered the same thing. I read recently that the way to approach them is to look for anomalies. Rocks that are unlike all the others; particularly large rocks, rocks shaped different from all the others. Because, well, think about it: if it's you wanting to hide something in your rock wall, wouldn't YOU think like that to help you remember where you stashed stuff?
Also, look for the entry way to the wall. Those can be tricky, because what looks like an entry might have been put there recently, not by the original wall builder (look for moss or water staining patterns for a clue).
One bit of stone I know you SHOULD check should you ever find one is a stone step, especially leading down to a cellar. People used to hide things under them.