Totally talking out of my ass here, but everything else that falls down into the abyss is randomly placed on a level below.
If we follow the same logic, the death backpack should be randomly anywhere on a level below the one you fell from.
PS: sometimes objects do hide other objects beneath them, if you use the magnifier lens you should see the whole stack.
PPS: if it fell on a tombstone, you have to defeat the tombstone first to get it from under the tombstone. I would check this possibility last, although wraiths can be easy to surprise attack in the doorways and around the corners. I would just assume the best at first.