[etc]

I found the water mark above the contacts for the battery -- it looks original, white with red stripes.

I think I was seconds away from the disaster.

OTOH, I had my 2 spare Li-Ion batteries go through a wash cycle in the washing machine - they were in a little plastic bag but got soaked pretty good.
Amazingly, they still work and appear to hold charge with no abnormal behavior. I assume no water got inside.