I couldn't think of anything particular for X other than X marks the spot. But what spot, you may ask?
Well, the Urban Dictionary says:
"What is said upon finding your target has been marked out. Derived from an X on pirate treasure maps.
The phrase was put into common usage by the British army, who performed executions by marking a piece of paper with a black x and positioning it on the heart of someone sentenced to death. The acting officer would say "X marks the spot" and the firing squad would shoot the x."
|X marks the spot|
I rather like the explanation of finding your treasure on a pirate's treasure map. I can relate to that on family history as I go on a search for "lost relatives" and then find a treasure of information. Don't you find that as well? Such satisfaction when a treasure is unearthed.