Char/ Ash in water heater

Wondering if anyone has seen this. This is the inside of the water heater burn chamber. There is this charred ash like substance. It looks and feels like burned wood. It crumbles and stains my fingers like wood.
There is a storm cap on the flue and gas seems to exhaust fine so I do not think it is from above.
This is in a seasonally used building.
Pack Rat? There is a good cover on the open burn chamber. I am recommending a new water heater with a flame arrestor, but this has me stumped.

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