There is a long thread regarding the gelating coating of Efke films at michaelandpaula.com forum. If you are interested look under the category "Developing film" and look for the thread "Uneven Stain". It might be of help. At least you will see that Efke films do have a gelating coating on top of the emulsion, and that they must be presoaked to get even results. I had seen my prints come up blotchy because of this very thing. Once I started presoaking the problem went away.
So who knows... It might be taht or maybe like you said it is a matter of contamination.