Stealth technology?
Bingo! Stealth technology is not that difficult, and may have been retrofitted on older aircraft (a skin of radar absorbent material, diffusion screens over turbine blades, etc.). You'd still get them on radar eventually, they'd just get a whole lot closer, maybe within cruise missile range.
Stealth technology?==
Exactly my thought.
Stealth technology?
Doesn't matter. Those airframes are so obsolete and poorly maintained they are no significant threat. Such flights were common during the cold war. Our intel community intercepted their communications from frag orders to engine shutdown. In-flight photos of them were taken by our fighter pilots. Stealth technology shields aircraft from radar but not other acquisition methods. Satellites and other systems maintain constant monitoring of the bases. The aircraft capabilities and their normal flight paths are well known. We do not have to know their exact location at all times, although we probably do. This undetected story is misleading. Old news is not news.
Hah, the TU-160 Blackjack, AKA B1ski, is about as stealthy as a cow in church. RCS is probably much larger than that of a barn. Larger certainly than the original B-1A. It's larger and has even less stealthy lines.
Plasma Stealth technology can be attached to any model aircraft. This would make old Bear and Backfire aircraft invisible to radar, and would require that avaitors stay up to sight the aircraft, like they did in the '50's before DEW was built.