Sounds like one of two things:

A college practical joke where someone bought a BD bike, bragged about it, and someone filled it up with sand to weigh it down.

Or

Check the seatpost. If it is open at the top, it might be that the bike has been ridden on sanded roads or a beach and the rear wheel has thrown up grit which has made its way through that hole at the top. This is the unlikely scenario.

I doubt that the frames produced by the factory for BD would be sandblasted.

Lucky that the BB is a sealed cartridge.