OK and I am 99.9% certain I have figured it out, but only from a few very well-placed clues, ie:

“You know who she is?”
“Yes.”
“And?”
“You really, really don’t want to know.”
“You’re serious.”
“Very.”

and:

But for all we know it’s been going on since the transition… or even before.

and finally:

“I just never thought she would… that she could… I mean, she doesn’t always… But this is something completely different. She doesn’t do things like this. She’s not that person,”

I will abide by the rules you have laid out for yourself very happily, as quaggy fic is worth waiting for !!