I had a batch of HP5 developed in Rodinal to show a lot more base fog than usual several months ago. I've been using HP5 and Rodinal for several years but never had this to happen before. My times and dilutions were the same as I have always used and the film was a mixed batch of 35mm and 120 rolls done in a large tank at the same time. I never figured out what happened to that batch of film.
David, what is eerie about your post is that I believe I shot all that film under heavy overcast, misty rain conditions over several days during a trip. Conditions similar to those you describe.