Perhaps this story has passed its 'sell by' date. The claims have been around for the past 30 years and nothing has shown up.
In contrast, the SR 71 broke cover within much less than a decade, during the height of the cold war, when security mattered.
Even the core design aspects of the H bomb were leaked within a 30 year time frame, despite diligent security efforts.
It may be that the intelligence bang for the buck is so much greater in digital snooping than in overflight that the resources were shifted, but the USAF still benefits from the mystery.