Lol. Why go on what Dani said himself? Let's speculate like we don't know what we're on about. Especially theory 1 and 3
1, Lucky shot? Try to hit a Radar Reflector (relatively static, just drifting with the wind) without an emitting SNR at Ashuluk.
2, Radar hack? No comments here...
3, The Invisible Man in the Rain... Probably the Serbs had agents at the end of the Aviano base with cellphones, they could calculate approx bomber arrival over Belgrade. (nothing else, just their SAMs were ready than)
4, How would you detect a stealth aircraft? I would use metric radar, like a P18 Spoon Rest, or even better a P14 Tall King. (do you know what was hit first during the conflict by a Tomahawk???