To sum it up, your Switch was docked and you've put the Joy Cons on the console as you left going whatever you were going to do.
Here's the thing: The Switch does not let you control the home menu when the Joy Cons are on a docked console. They have to be removed... so I wonder if that's how it happened...
Here's the thing: The Switch does not let you control the home menu when the Joy Cons are on a docked console. They have to be removed... so I wonder if that's how it happened...