No bees at all, not even a dead cluster—just lots of honey. Well, "lots" is a relative term. The total yield of hive contents came to approximately 40 lbs, lots more than I would expect in spring. The bees came from nearby colonies and were harvesting the combs.
Where did the bees go? Possibly uploaded into a flying saucer, possibly a honey bee rapture. Most likely they started dwindling in the fall until there weren't enough bees to cluster, although there should have been some dead bees somewhere. There always are in my dead colonies.