Unsurprisingly, combat (and other military) aircraft have the ability to turn off ADS-B, useful when someone who wishes you ill wants to know who and where you are.
It depends on what the aims of the mission are
If you are doing something to send a message then they leave it switched on so people can track them
For example - a few days ago a B52 flew from RAF Fairford across Denmark and through the Baltic and did a few circles around an island just East of Sweden
I think it is "quite likely" that Russia was watching it all the way - they would have been aware as soon as it passed a certain point anyway but having its tracking switched on meant that they were telling everyone else
including people inside Russia
AT other times I see Tanker type aircraft orbiting for hours over East Rumania or Bulgaria - or Poland etc. Now that is all the military stuff I can see - but you don;t have tankers flying circles for hours unless there are other planes around
e.g. fighters or spy planes that are there to protect "something else"
When the Global Hawk if flying around the Black Sea and Putin has been threatening them - then a tanker sometimes appears just inside the coastal area - which would be very useful to any fighters that were airborne to protect the Spy plane if Russia decided to sent something up to threaten it
I have also noticed that some U2s have been coming in and out of Fairford for some time - they seldom broadcast anything - but if they do then it is sending a message