Maybe it was just Cold War paranoia, they discover bodies laid out on the pass and under the cedar, well away from the tent, nothing makes apparent sense, they have seemingly 'fled their tent in panic' only to inevitably die outside, and due to the effects of frostbite and the cold the bodies have a strange pallor, so to process something they could not immediately rationalise they automatically thought it was "the Americans".
Back then nobody would consider chemical or biological weapons, so any mysterious set of mass casualties out in the open 'had' to be the result of radioactivity, and rather than having to send out for a geiger counter they would likely have this equipment on board military helicopters so used them, and picked up the radiation from rocks (which can also, with magnetic anomalies, affect compass needles).
Another possibility is that if the level of radiation later found on 3 items of clothing belonging to 2 hikers had somehow contaminated the tent, through proximity, it may have been slightly visible to the recovery team as a faint glow in the dark, in the manner of a luminous watch face. There may have been other radioactive clothing in the tent, items which weren't tested.
That was actually my initial theory when I was new to the DPI, that the hikers saw a glow inside the tent, perhaps on an especially dark night, thought they were pitched in an irradiated zone, removed clothing, freaked out and ran off, not returning, and that 4 bodies found in a ravine, most of them the best dressed, had been trying to decontaminate themselves.
Once the rumour of radioactivity grew, in the months before the ravine 4 were found, and if the earlier findings regarding Yuri K's clothing was made public before then, this would then explain the helicopter recovery pilots refusal to take the ravine bodies unless they were in lead-lined coffins.