[L fidgets; there's something inherently bothersome about disturbing a crime scene, he thinks. The moment someone lays hands on bodies or weapons, or the slightest little detail that can crack the answers wide open, it's all tainted. Important to number everything, photograph it, document the scene, before it gets dismantled by meddlers like him and his people.]

If it was wet enough in this chamber to freeze the water on the floor and their clothes, it would stand to reason that the moisture levels were high enough to rust the blade. That could have taken... perhaps five days? No less than that, I'd think.

[L enjoys it, too. It's when he feels the most like himself, fully inhabiting his body and mind without straying to doldrums or darkness. He lives here; it puts a light in eyes that otherwise appear deader than many corpses'.]

Ordinarily... I'd think that blunt force trauma was responsible for most of this. But this isn't frostbite...

[He indicates some burns on a dead man's cheek, white and sunken.]

Acid did this, and it also increases the concentration of hydrogen atoms. It would have speeded the rusting of that blade, in other words, meaning it's possible that we're looking at a more recent time of death.