Sand/grit from ventilation?

Weird problem. When I got the boat, there was what I thought to be sand in various area (no surprise), so I gave everything a thorough cleaning. Much to my chagrin, this strange sand like material keeps appearing in both cabins and the salon. A non-trivial amount, akin to someone covered in sand walking and sitting all over the boat. I'm baffled, but think this it mostly appears after taking the boat out. My hunch is that waves/pounding is breaking something loose, perhaps in the ventilation system where it blows out through the vents. Has anyone else encountered this problem?

Do you have dogs? Mine leave a similar amount after coming in, especially on wet days.

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No dogs, it's definitely coming from the ventilation. Someone suggested it could be oxidation on the coils, but that's the only lead.