“Did you manage to get hold of a big enough diamond?” Jesper asked.
Inej nodded and slipped the massive jeweled choker from her sleeve. “Hurry,” she said. “If Heleen hasn’t noticed it’s missing, she will soon.” Though with Black Protocol in effect there wasn’t much she could do about it.
Jesper snatched the choker from Inej’s hand, mouth agape. “Kaz said we needed a diamond. He didn’t tell you to steal Heleen Van Houden’s diamonds!”
“Just get to work.”
Kaz had given Inej two objectives: nab a big enough diamond for Jesper to work with and get herself into this corridor after eleven bells. There were plenty of other diamonds she could have stolen for their purposes and other trouble she could have made to attract the guards’ attention. But it was Heleen she’d wanted to dupe. For all the secrets she’d gathered and documents she’d stolen and violence she’d done, it was Heleen Van Houden she’d needed to best.
And Heleen had made it easy. During the scuffle in the rotunda, Inej had made sure that she was too focused on being choked to worry about being robbed. After that, all of Heleen’s attention had been devoted to gloating. Inej only regretted that she wouldn’t be there to see Tante Heleen discover her prized necklace was missing.
Six of Crows- Chapter 39 (Leigh Bardugo)
While I love this turn of events for Inej, there's no visible sign of it before this moment even in Inej's chapters.
We see the plot from the characters' POV, we're literally inside their heads during their chapters, yet up until this point we had no idea any of this is a part of the plan. We didn't know Inej got a special task. We didn't know she's looking for something among the guests. We sure as hell had no idea she's stringing Heleen along, or that the woman is likely to seek her out so soon.
But how do we get out? Once we trigger Black Protocol, you guys will be trapped on that island, and we’ll be trapped in the outer circle. We have no weapons and no demo materials.”
Kaz’s grin was sharp as a razor. “Thank goodness we’re proper thieves. We’re going to do a little shopping—and it’s all going on Fjerda’s tab. Inej,” he said, “let’s start with something shiny.”
I understand this is an effort to make the pacing more movie-like, but since the above mentioned applies, there should be at least a hint Inej got what she wanted, although perhaps not the way she planned.
Keep it vague if you need The Revelation to remain a surprise, but this total omission makes it both clumsy and it looks as if Inej were a robot, who can simply switch off a part of her mind and not think about what she's doing. What's the purpose of her actions.
The guards dragged Inej through the courtyard, past gaping onlookers, and out the ringwall gate as the bells continued to toll. They did not bother with gentleness or diplomacy now.
Inej didn't mind. She knew what's in store for her as she reached for Heleen's neck, and she got what she wanted anyway.
“I told you you’d wear my silks again, little lynx,” Heleen called from the courtyard. The gate was already lowering, as the guards sealed it in accordance with Black Protocol. “You’ll hang in them now.”
The gate slammed closed, but Inej could swear that she still heard Heleen’s laughter.
There's only one mention of diamonds before the chapter, where Inej conjures them out of her pocket:
Nina squinted down through the glass. “How does she[Tante Heleen] keep from collapsing under the weight of those diamonds? They can’t possibly be real.”
“Oh, they’re real,” said Inej. Those jewels had been purchased with the sweat and blood and sorrow of girls like her.
... but again- it's not specified the girls are talking about her choker. One would even expect some sewed-on gems for an occasion like this.
To sum up, there's such a thing as equivocation due to various characters' limitations both in amount of knowledge and understanding, then there's such a thing as hiding convenient pieces of information from the reader, when all of the characters should be informed, therefore unlikely to avoid thinking about said details.