I think it's a damn good job that most military aircraft don't appear on FR24. It would be like offering a seat at the ACMI debrief to every single intelligence agency in the world. Changes of heading after simulated missile shots would be quite obvious and repeated observation would allow our tactics to be analysed and countered.
Before anyone suggests that Mode S could be disabled inside segregated airspace like the MDAs to prevent such analysis, I'd point out firstly that collisions are most likely to occur between aircraft operating as part of the same package, so by the logic of combat aircraft needing a collision avoidance system Mode S should always be on, and secondly that we conduct much tactical activity outside of segregated airspace.