I just came back from an outdoor (subfreezing Minnesota) shoot, and processed a sheet of 4x5 (from a new box of FP4). The image has over a dozen clear pinholes. What causes these pinholes, so I can avoid doing whatever it was that caused it on this sheet?

Potential causes?
- dust from the camera bellows, (it's a 100 y.o. camera that hasn't been used lately);
- ice condensation on the film;
- some sort of chemical / pH issue (but it's boring old D-76 and Ilford hypo 1:4, with water rinse between baths);
- film coating defect;
- full moon...