A little gotcha I just discovered (if I’m diagnosing it correctly). When clearing a building, take care to drop extra rucksacks well outside the radius. Otherwise when you clear it (i.e. of all zombies and searchables) it will reset as cleared and delete the rucksacks.
At least, that’s what I think just happened to me. Lost two rucksacks that had been sitting on a porch for no more than a minute.