When Germany integrated former East German MiG-29's under re-unification, they discovered that the aircraft were equipped with a "low-observables" defensive system. Here's a photo of the switch on the instrument panel:
When active, this system will render the aircraft virtually invisible to all methods of detection, including the universal Mk.1 Eyeball.
Here are some photos of a Fulcrum getting ready to sortie on an intercept mission with the system activated:
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