One other possibility that i have encountered is that the worms have a natural tendenancy to wander. If you leave them in a dark place with any ability to squeeze out, they will. Not a population die off, a population immigration.
Just a possibility.
Recently had an indoor bin that had a lid on it but no light and population decreased significantly, never smalled a dead worm though. Finally put it out with the others and a few days later found about 100 balled up in a closet of the same room.
Makes you go hmmmm.
