Lets just imagine that Pippin was an informant/plant/mole/whatever seeking out the original rat to reestablish contact. Pippin knows he is seeking someone out. But he's never met this person and isn't sure who it may be.
All of a sudden while he's in the guard room, an
alarm goes off, and a person enters the room holding a
silenced pistol (which has to be out already because you don't hear any sound of a weapon being drawn). It becomes obvious to him, this is who he was supposed to be searching for ("It's you") but sadly it's too late and Pippin is toast.
In this instance Pippin could have met everyone and shook their hand in line and held a conversation with them. But if he wasn't sure who he was looking for it wouldn't make a difference if he had met them before or not. The "It's you" could be prompted by someone he's already met but has now revealed themselves as the Rat to him by coming in holding an assassins weapon.
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