"Okay, Grindelwald," said Tina in a strong feminist voice, "how did you find us?"
The iniquitous sorcerer's vile smile widened maliciously. "You were betrayed."
"By who?!" gasped Newt, wondering who would do such a thing. He didn't consider that he and Jacob might have been overheard when they were openly discussing their top-secret mission in the streets because that was done for aesthetic reasons and wasn't really part of the plot.
Anyway, Grindelwald stepped aside and revealed Mole McTraitor! Newt, Jacob, Tina, and Nagini were all shocked by this unexpected twist. Mole was such a seemingly unimportant character that they had completely forgotten about him, let alone considered that his brief appearance might be an important plot point.
"Mole McTraitor!" gasped Tina. "Who would've thought that you'd turn out to be a mole and a traitor?"
"I did it for money," said Mole, grinning despicably. "And let me tell you, it turns out money is pretty awesome. Do you realize that if you joined him, you could have money too?" It turns out Grindelwald got rich from subscriptions to his daily newsletter about cooking authentic New Orleans gumbo. This is canon now.
"We don't want money," Newt replied ethically.
"Excellent!" said Mole, rubbing his hands together. "Could I have some of your money then?"
"That's not what I meant, Mole!"