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The way you've set it up, with C being a trusting character, if you want the Reader to know something is up, you find a sweet spot of them saying something that most Readers would get, but C does not.

I'd suggest code talking. That is taken to comical effect in gangster movies, but code talking is actually a real thing in criminal groups; they intentionally talk about something innocuous that has a secret meaning between them.

In your case, maybe like this, as they are breaking up.

>Aaron said, "So that mission we were talking about, I can tell Dave you're on board?"<br>
Bob shrugged and said, "Yeah, I don't think we've got a choice."<br>
Cindy frowned at them. "What mission are you talking about?"<br>
Aaron laughed. "Gotta kill an evil Wizard! It's an online fantasy game we started playing."<br>
Cindy looked sceptical. "You are playing an online game?"<br>
Aaron said, "Ah, my son got me into it. Something to do together, you know?"<br>
Cindy said, "In lieu of talking?"<br>
Aaron nodded. "Exactly. You're full of insight into the male condition."