According to a number of reports, US military pilots are effectively transmitting their locations to the world during operations by neglecting to turn off their transponders. This allows a whole host of websites and apps to track their flights, potentially giving valuable or sensitive information to enemies who can then check where the fighter jets are heading.
The information given off by transponders that have been left on include their exact location, speed, and altitude, meaning that anyone can see where a pilot is and their likely destination or arrival time. While this isn’t a problem for most flights, those carrying out covert missions or special forces operations would be expected to go dark so as to not give away their position.
All aircraft use transponders and they act as a way for them to communicate with other vehicles and air traffic control. This allows them to be identified quickly and easily while also serving to help prevent mid-air collisions as they are usually integrated with avoidance systems. However, military aircraft carrying out operations, especially if they are involved in special ops, would usually turn off the transponder to ensure they remain undetected.