I would agree that a mouse – or their family – was the way the vermiculite went into the exhaust. Not sure where it came from. If it was in the garden to soak up water for the plants, then it is OK. If it used as insulation, it may contain asbestos if it was installed before 1978 when it was banned for the health of the installers who inhaled the dust.
You have cured the problem by welding up the front door of the mouse’s entry way, so should not be an ongoing problem. Best of luck with it. Herb Tull