I think

Fish homicide happened in our work tank. I am 99% sure of it.
Why?
Because when I left on Friday we had five fish and now there are only four.
Missing?
The mangy-looking guppy.
Culprits?
The fat little platys no doubt.
They started out all sweet and innocent and quickly grew into giants (ok not giants, as they were still smaller than the guppies!) thanks to my coworker's overfeeding during my sick time from work.
They seemed a bit nippy. Were they biting? It looked like the black-tailed guppy had a chunk out of his tail. I thought maybe it was ich, but none of the other fish had it, and the symptoms weren't quite right. Then I thought it was fin damage due to water change, and planned to remedy the situation with an aquarium salt regimen I found on a guppy-breeding website.
But then today, he's gone. Just like that!
I picked up the cave/rock decoration he used to like to hide under, and nothing. Weird bits of stuff came out of there. Was it his remains? I don't know - nothing was intact enough to make a positive ID.

He's not in the filter. He didn't jump out of the tank.
He disappeared.
And I think he disappeared into the fat bellies of my two little seemingly innocent platys.
Little bastards.