Please look at the example of fogged film in the technical gallery. It is HP5 Plus sheet film with bands of exposure. It affected about 15 of the 100 sheets in a box of film. It fogged both exposed and unexposed film. I have not seen this type of fogging before.
It should be noted the film was shipped via UPS, and went through two US commercial airline flights before development.
If the folks at Ilford are lurking hereabouts, I believe the emulsion batch is 77BHK1CO2/06-7, expiration date November 2005.