Clue 1: In the last chapter of the first book of Unstoppable, there were two strange events being the whiskers being stolen at the museum Dan and Atticus went and the fact that Amy thought she saw the Anatolian leopard (judging by the bright green eyes she saw) but was mistaken because "it wasn't the impression of a leopard's paw, it was a boot."
Clue 2: In the page of 'Flashpoint', Gordon said, "As it turns out, old agents never die; they just wander off to top-secret undisclosed locations to spend their share of Grace Cahill’s money. That is, until the next crisis requires us to jump back into the fray."
But the question remains, who is the hidden agent? I suspected it was Irina, however, there's one thing I have to confirm: was her body found in that island? The island was to be converted into a secret Ekaterina stronghold (or was it a resort), so her body could be found by the construction workers or something. Irina could be the hidden agent since he's the only one who could pull off a heist perfectly.
But then, Clue 2 will narrow down the possible identity of the agent since Gordon mentioned 'old agents' and 'spend their share of Grace Cahill's money'. But, do we actually know everyone who went to Grace Cahill's mansion that very day?
I'm sorry if this could annoy you in any way, I couldn't shake off the fact that there's someone secretly helping the Cahills. There could be another clue in the third book, maybe not.
Please post your answers/thoughts/comments here! Well, I know I'm not the only one wondering about his/her identity.