Squawking 7500 signifies an aircraft being hijacked and could result in an aircraft being escorted by military forces.
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7000 is a code that VFR traffic will transmit so they can be seen by ATC stations. They may not be talking to that ATC , but if they can be seen by them, that ATC unit can vector other aircraft that it is talking to out of the way of the 7000 aircraft.
Code 7400 may be transmitted by unmanned aircraft systems (UAS) when the control link between the aircraft and the pilot is lost. Lost link procedures are programmed into the flight management system and associated with the flight plan being flown.