I spent 2 years stationed in Iceland chasing Bear bombers and it wouldn't surprise me a bit if they were able to overfly Iceland pretty much unnoticed since there's no military presence ther any more.
All the Icelandics have are three radar sites (assuming they are all still operational) used by our military previously but that are now used only as Air Traffic Control facilities. If the aircraft weren't squawking there's a good chance the Icelandics wouldn't see them even though they would get a raw data return from the aircraft. Raw data wouldn't tell them who they were, only that there are aircraft there.
So why do you think they're flying over? Are they testing something?