Deduces fake key was already in safe

“What do you think happened back there, ma’am?” I asked.

“Evil,” whispered Ana, “and trickery. I think the twitch came looking to
steal the reagents key you found in Aristan’s safe house. Yet they were in
for a surprise…for after killing Nusis, they found that the key had already
been stolen, with another key left behind---a fake. The one you just
handled.”

“What?” I said, stunned. “You think tha—”

“Keep your voice down!” she hissed. “It’s now clear that there are
many spies among the Iyalets here! I cannot give the game away just yet.”
I whispered, “Do…do you really think so, ma’am? That there’s a…
what, a thief about?”

“I do. Someone must have learned what Nusis had in her safe, snuck in,
opened her safe, and took it, leaving another key in its place. Then the
twitch came, forced Nusis to open her safe, then killed her…yet when they
seized their treasure, they realized it wasn’t the right key. Someone had
beaten them to it. The twitch then understood they were in a tricky spot—
robbed of their prize, and now fearful we might now realize the true nature
of the thing they sought. So they carefully shut the door and wiped it with
alcohol, removing any blood, hoping we might not deduce what could have
been worth murdering and robbing poor Nusis over.”

I tried to think through this, my mind spinning. “What makes you so
sure the twitch themselves didn’t leave that key as a fake?”

“Because they went to the trouble of cleaning both the exterior and
interior of the safe after they killed Nusis. The twitch is methodical, and
careful—they don’t normally make mistakes that require cleaning. So…
why would someone so careful manipulate and clean a safe after they’ve
already robbed it? Why create this mess for themselves? The easiest answer
is that they did not get what they sought and wished to hide that they’d ever
sought it at all.”

- The Tainted Cup, Chapter 36 pgs. 323-324