>>There are now no queen cells in evidence anywhere.
Bees will often begin to build queen cells at various times due to various stimuli and then tear them out again before using them for no obvious reason to the beekeeper. I would say that was probably the case in the circumstance cited.
>>I observed capped brood and larva ABOVE the upper slatted rack.
I use 4 medium boxes for my brood chambers, If you are too then I refer you to my often made statement to allow the queen as much room as she wants, needs, or demands. WIth every box the same size (medium) moving those frames down into the brood chamber and moving honey frames up is easily done. Normally 4 mediums is as much room as the majority of queens need but there are exceptions--you may have one.
I'm glad to hear you like my DuBray Slatted Rack.