When in the book does it show that Sir Percy can be the Scarlet Pimpernel besides him being in the room at 1 o'clock?

Percy Blakeney keeps his identity pretty well hidden. He's kind of like Clark Kent, sort of nerdy, but turns into Superman when he needs to. There aren't any phone booths to change into the Scarlet Pimpernel but Percy seems to do okay. I suppose a clue could be the guy is gone every time people are smuggled into London. He also seems to be in the thick of things but always backing down or refusing involvement.