Just as the title says:

After killing a monster on terrain that is quite hilly, or sometimes after killing a monster (usually waste collectors) in a mission, the remains of the monster will either sink below ground level or will end up inside a wall. This results in the remains being unlootable.

Does this happen often? No. But it's there, and smacks of a certain lack of polish, or "we're not-quite-out-of-beta-yet". Please fix this, as it's one of the first bugs a starting character will see happen.