Microwave ovens are crazy powerful things. I heard that the stealth bomber shot down over Serbia in 1999 was revealed not by radar, but by dozens of cheap microwave ovens retooled to emit focused beams that swept the sky. The electromagnetic waves in a microwave oven are specially designed to excite any polar atom or molecule, unlike microwave radar that's only used to illuminate a target. It's sort of like the difference between a bat's echolocation and a sonic drill... same stuff, wildly different effects.

Anyhow, the retooled microwave ovens had a short range (atmospheric water vapor would absorb them), but it was long enough to pick up the low-flying stealth bomber that night. The bomber was designed to absorb microwave radiation (instead of reflecting it, like most metal objects), but the consequences of doing so were quite different for microwave oven radiation than for microwave radar radiation. The plane's coating became so powerfully excited it started radiating infrared radiation of its own, which made it ridiculously simple for SAM batteries to acquire, track, and shoot the bomber down.

Of course, that's the sort of trick that only works once, because it's as easy to fix as sending in F-14s or cruise missiles to blow up anything that emits any kind of microwave signature from outside its detection range. Only the hubris of relying on unsupported stealth bombers led to one being shot down, a hubris that the Serbs were quick to deflate with signs reading (in English) "We're sorry, we didn't know it was invisible!"

Aviation Week has dropped dozens of hints about just what happened in the 117 shootdown, but (in the issues I received anyway) never came right out and said it. From what I've gathered, including what you say here and what I just found on wikipedia, the Nighthawk crews got cocky, flying the exact same route at the exact same time at the exact same altitude every night. The Serbs (perhaps via the IR trick you refer to here) eventually sussed this out, and at the right place and the right time fired a brace of SA-3s blind. Using a stop watch, they switched the SA-3s seeker on so close it could even pick up the 117, and then boom!