We know, thank to some websites like datadrivengamer.blogspot, that your party can only obtain a chest if they fought monsters in a treasure room (and only if this treasure room was seeded with a chest at level loading).
But some of us have experienced having obtained a chest ina random encounter in a corridor.

I have formulated a theory why (in most/all cases) :
- Your party goes into a treasure room seeded with a chest and encounter friendly monsters,
- a have flag is set "this encounter will give a treasure chest",
- you choose not to fight,
- the flag is not unset,
- you wander in a corridor till you have an encounter and fight the monster,
- and, if you win, you will be given a treasure chest,
- the flag is unset.

I have tested the theory on level 10 where every room has monsters (sometimes friendly) and a guaranteed chet (unless you fight Murphy's Ghosts).

On another note: if you save your game and return to the Title screen (or quit the game), then reload that save, all treasures rooms will pass through the process to seed them with trasure chests anew.
It is a method that I use to farm level 10: go to level 10, walk up to the first door (don't enter the room), save&quit to Title screen, make a copy of the save, then reload the save, do all the rooms save Werdna's, use left teleport to go back to the begining of the level and if the party state permit it, walk up to the first door (don't enter the room), save&quit to Title screen, make a copy of the save, then reload the save.


I have also completed my bestiary the only two entries not completed and not obained are "Vampire Lord" and "Werdna".

That's true in other versions as well.

I've also noticed something interesting with the alarm on level 4. If you go through a door immediately after triggering it, the resulting encounter will award a treasure chest, but if you instead walk down the hallway, you won't get a chest. (Tested in 0.1.x IIRC.)