Comments as per your suspicion: Sure, it could have been an old queen that suddenly died/disappeared. That certainly happens. Good old queens though are usually `slow`, she looks good and she has a good pattern just not as much of it as the next hive over. Failing old queens will produce a spotty pattern as she runs out of sperm or runs out of capability of controlling it when she lays. Either of those scenarios the beekeeper usually picks up on by observation. So do the bees, they will usually start supercedure process. Sometimes the old queen ticker stops before the SS proceeds to its end, where she just suddenly disappeared. Perhaps that is what happened, and there is a SC queen in there somewhere, yet to be mated.
About queen ages - An OLD queen nowadays is one that is between her 2nd and 3rd year. We should try to know the queen ages and anticipate a need to requeen each hive in the queens 3rd spring before she is failing and before the bees replace her themselves. We do this by marking the queens or a tag or tack on the front of the hive following the international colour code for the year. By seeing a marked queen and the colour of the mark, we know her age and can plan her replacement. Requeening by the beekeeper is a controlled event. Requeening by the bees is an uncontrolled event. Pick whether you like to be in control or out of control of the hive.
Queens also can and do get sick from pests, parasites, diseases, and other chemically charged stuff that the foraging bees are bringing back. Just like every other bee in the hive. This is the explanation given as to why queens are much shorter lived nowadays than the golden years of decades ago, and also why some queens just suddenly disappear. So put that on your list as a possibility as well.
Swarmed? There should have been alot more going in the hive that would have given you clear signals to make you be thinking about a swarm event well ahead of time. You had that experience last season. Did this hive look anything like the one that swarmed away last year? Meaning brood, bees, combs, resources, cells, etc. Did you miss it, or is there something else that occurred such as an old queen or sick queen disappearance.
For your considerations.