I am surprised military aircraft show up on FR24, security risk.
Not all of them do - it's usually the transports and training flights that show up, plus anything connected to a display or a flypast.
I'd agree it's surprising that some of the reconnaissance aircraft and tankers on active missions do show up, but in the case of the Black Sea operations it could well be a deliberate move to make it more obvious to the Russians that these are there so that they don't get shot down and cause an escalation?
I’ve always presumed that it’s very deliberate; the question is what isn’t visible. The answer is, I suspect, quite a lot!
Regarding military aircraft.
Firstly you have to understand what FR24 is and how the website obtains its information.
It is not a radar display.
The information is obtained from aircraft carrying certain mandated transponder types, which hobbyist’s can pick up on their receivers and sent (via the internet) to the FR24 site.
There are still lots of transponders in use, that do not need to comply with those civilian regulations.
Also, where enthusiast receiver coverage is limited, the number of aircraft shown will be limited, or non-existent.
Actual, civil radar displays, show all military aircraft operating.
For example, in the UK and the rest of Europe, civil air traffic controllers will see everything flying within radar coverage.
Military aircraft not showing on FR24 will be perfectly visible to civil ATC.
Is the right answer. They're visible because they want to be. Either because what they're doing isn't particularly sensitive so it's an aid to navigation (so everyone knows where everyone else is both military and civilian) or they want people to know what they're doing as a statement of intent. See the recent B-52 flights within a few miles of Kaliningrad and along the Gulf of Finland. Those were visible because the US wanted everyone to know exactly what they were doing. Same logic applies on Marine Traffic (the water based equivalent of FR24). Warships show up on civilian systems either to make life easier for everyone or because they want everyone to know where they are as a statement.
It's a matter of a flipping a switch (or probably pressing a button or two) to turn off the transponders that allow for such things to be tracked on civilian systems.
The only time civil systems will not display military radar tracks, is either when they are flying at very low level, below radar coverage in a particular area, or on active live combat operations, where they’ll go dark to civil secondary radar and similar.
Primary radar returns can still be seen, unless the aircraft has stealth characteristic, employing electronic countermeasure, or again, flying below radar.
In combat situations, primary radars would be electronically jammed.