There are ACARS 'spotters' (enthusiasts) who monitor ATC, and track all overhead flights depending where they are located and add them to there movement lists.

Basically there are very few areas in the USA that somebody isn't watching, and I might add, there are afew ATC staff who their work is also their hobby and fill in the blanks when required.

So if there was a specific aircraft that somebody, could be a repo company, press, propublica or whoever, it wouldn't be hard to find. You might miss the odd flight, but generally it isn't too hard to find.

This aviation groups as a whole, also have access to multiple sites that fill in the blanks, just a matter of knowing where to look and filling in the blanks.