Repost per anons' request

We have seen mil a/c with ADSB on then suddenly off. We don't know for sure their reasons for being seen or unseen. Over CONUS, ADSB receiver coverage is quite good, so if a plane disappears it likely turned its transponder off for a reason. Or the military could be performing electronic warfare countersurveillance operations that causes ADSB transponder messages not to reach the database.

Europe has good ADSB coverage. Elsewhere in the world, ADSB coverage is very spotty and planes disappear because there's nothing to receive and forwar their signals. The ADSB database we use consists of volunteer-owned receiver equipment forwarding messages to a web server.