[Uncle Peter]

Military aircraft do not normally broadcast the protocol which identifies position, altitude and speed (ads-b). Actually neither do many private aircraft, executive jets and short-haul prop liners

There are ways of tracking such aircraft by other means but it's unlikely that you will see the military ones on FR24.

There's a NATO air exercise in Bulgaria next week so you might have seen F16s/F15s air-to-air refuelling en-route