@MartyCormack is there a hole in the tire, that the grit and debris got in through, or was the pressure very low and it got in around the valve, or from a messy tube change at some point? Some bits of rubber wear off of the tube and tire, if you had sand and small rocks, I would suspect there is a cut in the tread
@enobacon I inspected the tire, a Schwalbe Marathon Efficiency, puncture resistant, no holes or cuts. I last put it on in a clean environment. The valve hole is the only thing I didn't check. Hmm...
@MartyCormack what was the grit and debris made of, definitely not parts of tire casing, rim, or tube? The rim tape should be keeping stuff out if it's sealed over the spoke nipples and the wheel never sat flat in storage, but maybe it smuggled something in from where the rim was sitting before mounting, or there was static cling to the tube.
@enobacon The debris was black, it could have been part of the tire? The grit was probably sand. I checked the rim tape, it looked good. Maybe static cling?