I had seen a leaking button cell battery, but never found a jumping one. The AG10 cell on the left (picture) was found 20cm away from its cousin, that's where it was put to rest after removal from a toy pen.
It felt fat and round, and still gave some energy to power my Joule Thief, but then overnight it probably exploded:
There is no leak of any substance and the process has apparently stopped by now. Other two specimen removed from the same toy do now show the same failure signs.