Not sure what's going on there, there's no staining at all in the other side of the paper (inside the red circle):
In my case at above Panasonic 9V, the corroded battery's snap terminal looks like caused by corrosive gas/vapor that was vented slowly, no leaked liquid mark/stain at all inside.
Maybe, but I never heard of "corrosive gas" from batteries before.
Me too, but how do you explain the severely corroded terminal at the tab, and also happened from the inside the battery's terminal ?
I re-attached bigger tab photos below, so you can see more in details.
The corrosion did heavy damages at the nickel plated outer case, and also the brass inside.
Also from both terminals position, as I stored the Fluke lie down on it's back as usual, and the corroded one (negative) was on the top, while the good one (positive terminal) was at the bottom.
Fyi, I have OCD, and I'm 100% confident that I didn't miss spot it, if there was a corrosion at terminals when I installed it, and clearly I would replace the tab asap, or stop the battery installation.
The black goop thingy at the bottom in the photo, is just a hardened, darkened & crushed foam, for preventing the battery from rattling, and its not the culprit as I have many similar hardened foams else where too.