Hive Collapse

Dakota18B colony has completely absconded. Hive inspection up until now have shown a healthy but average sized colony. Hive was full of resources and not diseased. This represents the 3rd time in 2 years that a commercially sourced bee colony has for some reason just disappeared. There are a fair number of dead bees left in the bottom of the hive but not nearly enough to account for the entire hive. I have seen this exact same scenario 3 times previously. Hive body full of resources. Hive treated for mites with oxalic acid. Sometime in mid to late fall, entire colony just disappears.