Maybe they didn't have enough time? It's starting to look like no matter how the murder was done, it'd have to be fast.

Then we're looking for one sloppy killer. If I was planning on murdering someone, I'd make sure I had every last detail down exactly right before making my move. [He chews on his thumbnail.] And what would they need it for? Cleaning supplies...if we were looking at a human corpse, I'd think they'd want it to try and clean out any blood that might have spilled, but Rin's a robot. Was a robot. There'd be no point.