I have encountered several US-registered planes (tail numbers starting with N) which do not appear to be in the FlightAware or FAA registry. All of these planes have been flying for several years and some of these planes fly frequently. Here are some examples:
N340GZ
N993QS
N556MP
N211HV
I first through this was due to the planes being on the blocked planes list, but there is owner infos for the couple of blocked planes I checked:
Mark oops yes you are right. Here is another which I knew flew IFR in at least 2009 and I believe is currently flying (it is on the blocked list):
N802DR
More generally am I correct in thinking that US-registered planes with unknown owners are a temporary phenomenon (say due to change in ownership or reassignment of tail number) which would be resolved in a the next update or two of the FAA registry?
N211HV was a King Air 200 re-registered N102FG in 2005. Not a current registration.
N340GZ no idea - no a current registration.
N556MP was a Piper PA-46 registration cancelled in 2004. Not a current registration.
N993QS currently a reserved tail number.
All four registrations you have listed are not valid registrations, the database is updated daily, all four are not in the current database, and even if re-registered recently - would still be there.
As for N993QS that is a reserved tail number - so someone (James Morris of Omaha NB) has payed a fee to reserve this tail number.
It is possible they are old expired and no longer valid tail numbers.
If you encounter any of the above tail numbers - take a photo - you can also contact FAA or local tower to inquire.
I asked a similar question of the FAA aircraft registration people, regarding an aircraft seen in a commercial. Was told the FAA will approve a bogus number for commercial/film work.
I have time in 340GZ. It was registered to my old boss that I worked for in TVC. GZ was his initials and turns out the new n-number registration never went thru so the number on the side was changed back to 340VF. Later was sold and became 340GB. Not sure where the bird is now!