My assumption is that the mold occured after the bees die because the natural convective flow that would have normally ventilated excess moisture ceases with their death creating conditions that allow for mold growth especially toward spring.  

Interesting thought Bill,  I came to a different conclusion though  and here is my thoughts.

Dead outs with a  small cluster of moldy bees is probably exactly what you said.  They died from something else.   This would be different from the box that’s opened and the combs are all moldy.  I suspect they died from moisture,  the reason being that if the hive had normal moisture levels there would be zero mold.  For this I offer up literally thousands of stacked supers and boxes with drawn comb,  they sit here in the yard with nothing but covers on,  all winter,  and zero mold.  (let a little rain in a fresh comb though and watch how fast it molds)

So my assumption is/was that the bees in the box continued to metabolize water,  but for whatever reason were completely unable to remove the moisture from the box.  This lead to a slightly warmer than outside box  with super saturated humidity.

To your point though  that is completely different than the small path of mold which accompanies a dead cluster.

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