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There's a nice technique for this: have the character notice something is off, then either have them **dismiss** it with an unconvincing explanation, or **distract** them. This can be combined very well with Amadeus' answer for code speaking, either by having the character notice something off about the code or by introducing the code talk afterwards, when the reader is already alerted.

## Dismiss

> "Nice new wheels, by the way," C said. "You finally saved up enough to replace that old junker, huh?"
>
> Knowing how much A loved to talk about cars, she'd meant it as an easy conversation starter. But there was a strange silence as A and B shot each other an indecipherable look.
> 
> "Yeah," A finally said. "The money just added up eventually, you know how it is. So what have you been up to, anyway?"
>
> Inwardly, C frowned at the abrupt change of subject. Knowing A, she'd expected him to jump straight into a monologue about the exact specs of the new car she'd seen outside and where he'd gotten it. This wasn't typical. Was he not telling her something?
>
> Or perhaps her imagination was running wild. These were her oldest friends! If something was going in their lives, they'd let her know. There was no need to start fantasizing just because A didn't feel like talking today.
>
> And since he didn't feel like talking, it was her turn to fill the conversational gaps. Better hop to it.
> 
> "Oh, you wouldn't believe what happened to me at the office the other day..."

The reason this works is that your character isn't aware they're in a story, and so "I must have imagined it", "he doesn't feel like talking", "coincidence" and the like seem like perfectly reasonable explanations. The real world is messy that way! Your _readers_, however, are aware they're reading a narrative, and that there's no way you'd draw attention to something like this if there wasn't something more to it. Nor do your readers have years of trusting and loving characters A and B - to the contrary, they're primed to expect conflict, and a sentence like "If something was going on in their lives, they'd let her know" is just begging for dramatic irony. So you've effectively let them know "hey, something is off here" while sliding it past the POV character.

(Incidentally, having the POV character dismiss something like this with a _convincing_ explanation is a good way to foreshadow a plot twist you don't want caught on the first read-through... but that's another answer to another question.)

If you really want to make sure nobody misses it, have the POV character mentally return to the scene later and dismiss it again. An extreme example:

> C left the house with a spring in her step. With everything that had been going on lately, it had been so nice to spend an afternoon just catching up with A and B - they'd have to do it more often.
> 
> There had been the odd moment earlier, though. It had almost seemed as if they were hiding something from her...
> 
> Just her imagination, C told herself firmly.
> 
> _chapter ends_

This is the equivalent of waving a bright red flag at your readers going "it's not just her imagination!" Note however that this forces you to make the payoff bigger (more on that below).

# Distract

> Inwardly, C frowned at the abrupt change of subject. Knowing A, she'd expected him to jump straight into a monologue about the exact specs of the new car she'd seen outside and where he'd gotten it. This wasn't typical. Was he not telling her something?
> 
> A crash interrupted her train of thought. B had started fiddling with his cup while she was distracted, and now it had slipped out of his hands to smash on the floor. Hot coffee was everywhere.
>
> "Oh no!" C jumped to her feet. "Are you all right? You didn't burn yourself, did you?"

Overall less versatile than the above, IMO, and can come off as unbelievable if you don't do it right or do it too often. It can make a nice change, though. It helps if you show your character similarly losing their train of thought in other situations - in my writing, I use this most often with a character who's generally very distractable and forgetful.

Also, depending on how big the distraction is and how long it takes to resolve, you may also make some of your _readers_ forget what was going on.

# The payoff

This technique is basically a Chekhov's gun: you're showing it to the reader, and now they'll expect it to go off. The bigger a deal you make of it, the bigger the resulting payoff needs to be. For instance, the extreme example I showed above - where the character repeats the notice/dismiss cycle right before a chapter break - basically forces A and B's criminal endeavours to be a significant plot point and for an eventual dramatic reveal to C in order for it not to come off as overhyped or anticlimatic.

# POV notes

Unlike Galastel, I think this technique can be used to great effect in both first person and third limited. Having your readers know more than your POV character can be an excellent source of ongoing tension ("when will C finally realise that A and B are up to no good?!"), give rise to entire arcs (imagine C being increasingly torn between her trust in her friends and what her observational skills are telling her), and allow for some nail-biting suspense scenes if you put your character into danger that the readers are aware of but they are not. You do have to be a little careful if you're using it repeatedly to make sure your characters don't come off as unrealistically naive or unobservant, but you can work around that.