Air Force One doesn't respond to ADS-B or MLAT queries so it can't be tracked. You won't find AFONE on any flight tracking website for that very reason, just like you won't find any of the other planes that don't want to be seen (U2/SR71/whatever secret replacement is being used now).
The technology that brings this service isn't all that common yet (despite the websites claims that it is). Airliners will likely have it, regional jets and smaller likely won't unless they routinely fly over unmonitored areas (Canada's arctic or across the ocean for example). A handful of private jets do have it as well though.
To resurrect the dead anyone else notice that flightradar24 doesn't provide information on military flights. You see something fly overhead and wonder where it is going and it is completely and totally "stealth."
Military flights don't respond to ADS-B or MLAT queries, even when equipped, except under specific circumstances. In some cases, they may respond but have specific agreements not to be tracked or monitored, and therefore won't show up in any applications (Canada's government flights, for example, do have ADS-B and MLAT so the military can keep an eye on them, but they aren't tracked or monitored by anyone else and won't show up on FR24 or FA).
Might not be an aircraft. Could be a drone. Apparently, there are now over 7,000 (more or less) privately owned drones and they are becoming a hazard to real airplanes.