I emailed Rob and he said that the positive contact may have broken and slipped down to make contact with the battery. Gave me a suggestion as to how to remove the spring screw and offered to repair it. I'll need to be more careful in the future. Like you said, never leave a battery in it when it's not in use. I should have done my homework.
The old saying is that "you can never be too safe", but I think that removing the battery from a mechanical mod when you go to bed at night is excessively safe. If your mod is in good working order when you set it down, there is no reason that it won't be in the same state when you pick it up later.
Unless you have gremlins.......I hadn't considered that.