I am a taxidermist, and it sounds like plain old dermestids to me. There must be different species of dermestids, because the ones you get in mounts aren't the large ones like JJ spoke about. They are small black beetles, and they can fly. They lay their eggs, then the larvae eats the mount. You get frass (sawdust looking droppings) coming out of the mount where they feed.

I have had them show up in European mounts (skulls) years after they were boiled and bleached. It only takes a tiny bit of meat for them to snack on, and Europeans usually have some meat left in the sinus area. That is one main reason I hate skull mounts, they can be a catalyst for your whole trophy room getting destroyed. Another place bugs like to start is under the sheaths of horned animals like antelope and sheep that weren't cleaned properly.

Usually dermestids don't eat tanned leather. I am wondering if there is some residual meat left behind on the skull cap area?

Here are pics from a mount I tore apart last summer. I had it mounted by another taxi in 1991. I never knew it had bugs until I tore it apart. And none of my other mounts ever got bugs. I also found that the leather itself was not touched. They simply burrowed in under the mache that covers the skull, and started snacking.

The yellow debris on the piece of wood, and right next to it, are the shells from the larvae.
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Note the uncleaned skull cap! This is what attracted the beetles.
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More membrane under the skull that wasn't cleaned:
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