In recent flooding I started excavating our (cracked, failing) concrete basement floor. (it's just a thin layer over a dirt floor--old house)
I stopped when I found a 1/4" copper line buried in the concrete. It's heading toward the heating system. But that was upgraded to gas some time ago, and I can clearly find the gas lines (they're overhead). Doesn't seem like electrical conduit. Doesn't seem like water pipe, but...who knows.
Any idea what this might be? Why would a tiny water line be heading to the heater? For now I've marked it so I don't disturb it.