A properly soldered, wiped and taped splice is a perfectly good thing to find behind a wall. There are hundreds of thousands of them behind walls today in homes across America.
When the solder "falls off" then the journeyman or apprentice that day made a mistake and cold soldered the joint. While there is a greater responsibility with K&T during install, this does not mean that a properly done installation is a risky one. There are even arguments that would say it is a safer method due to the inability to drive a big screw into the wall and short both leads.
The assumption is that the install was done correctly and no one has tampered with it in the history of the home, which are big assumptions.
When the solder "falls off" then the journeyman or apprentice that day made a mistake and cold soldered the joint. While there is a greater responsibility with K&T during install, this does not mean that a properly done installation is a risky one. There are even arguments that would say it is a safer method due to the inability to drive a big screw into the wall and short both leads.
The assumption is that the install was done correctly and no one has tampered with it in the history of the home, which are big assumptions.