Originally Posted by
evamodel00
(I'm no avionics/airman expert by any means so please educate me since this is more of a question), If the aircraft had altered course and crossed over land, wouldn't a country's radar picked up the aircraft and had fighters/local craft investigate it if the craft didn't respond/wasn't scheduled to be flying there?
Depends on where and how high. An undeveloped area (ie jungle) not frequented by aircraft may not have ATC radar coverage.
Radar coverage is more limited at low altitudes.
Also with the transponders off, a primary return might just be random GA aircraft (some countries allow that, some don't).
Just because an unknown aircraft is flying around, that does not mean the military goes to DEFCON One. This depends on the country, in China an unknown aircraft would be intercepted for sure. In the US, we have thousands of unknown aircraft buzzing around as we speak (no flight plan required here, and a transponder is only required near a large metro airport).