Unsure if already found, but I haven’t seen anything regarding it as of yet.
Anywho, to activate, you’ll want to be playing as Richtofen. Near the electrical tower, there will be a barrel with faint sounds of a woman screaming. Melee the barrel, interact with it, and then interact with it again. This will pop you into the cutscene.
I won’t spoil the rewards, but the story is awesome!
Also, ELEMENT 115 MENTIONED???!!
EDIT: I have just seen two other people who have gotten similar cutscenes but not the full thing.