Interesting activities going on across Europe in terms of the monitoring of the Russian forces on the Ukrainian border.
As it was pointed out previously, you can only see the transponders of the aircraft which wish to display, the rest is falling "under the radar", per-sey.
This is an airborne ground surveillance system for monitoring troops. Currently side-on with Moscow and aptly named "REDEYE".
This is an air-refueling tanker, Boeing KC-135R, one of many in the air. Many appear to be flying up to the North Sea where there are likely to be Boeing E-3 Sentry (AWACS) monitoring and acquiring air and sea targets across Europe.
This is a Boeing E-3 Sentry (AWACS).
U-2's have been reported to be flying out of RAF Fairford, but they aren't broadcasting on MODE S (ADS-B).
A Reaper drone, possibly out of Cyprus, was observable over Eastern Romania until about 20 mins ago, but it went silent on approach to the Ukraine border.
If you want to watch on ADSB Exchange, click the U in the top right corner to filter out the non-MIL aircraft.
Only thing there that isn't usually out on the eastern front is the J-STARS.... There is an American deployment due this week or next as a show of commitment to NATO, where something like 10,000 troops will be flown in for an exercise, so probably something to do with that...
But on a regular Mon - Fri you'll have:
1 or 2 U2's out from UK
1 or 2 RC-135 up from the UK
3 or 4 E-3 AWACS up somewhere
1 or 2 RC-12's up from Estonia on the Kalinigrad boarder
You'll have something similar up with assets from the Med going to the Black sea
Everyone loses their **** when the Bears or Blackjacks go for a spin (albeit, they don't plan flights so do it in a less safe manner)