Commenter "out" left a suggestion as to why I may have lost one of my hives (hint: not CCD). Noting that in North America, it's not a question of whether you have mites, but rather how badly you have mites, he suggested that may be my cause:
the clue that you gave was that the dead colony was a boomer last summer/fall. the more bees you have = more mites you have since they are parasites.

when a hive collapses from a mite load you find no bees. if i am correct you will may find some mutated white larva on bottom board that was killed off by the mites and removed by the bees before collapse. the larvae under magnifcation may show mutated wings

in a heavily mite infested hive the collapse happens as brood rearing winds down in early winter. since the mites have no larva to hide in and replicate the whole mite population is not living on the honeybees.
He went on to suggest that I try "powdered sugar" to test for mites. A Google search later turned up this article on how. In a nutshell, you put a couple hundred bees in a mesh-covered mason jar with powdered sugar. The powdered sugar makes it difficult for mites to hold on to the bees, so when you shake them, they fall through the mesh and can be counted.

I'll be giving it a whirl this week, and I'll let you know what I find. Thanks Out!