They may not count as rare yet, but Falcon 50s are far from common at this point.  They have been long out of production and the owners have progressively traded up to something newer.  Consequently, when one shows up, it is a nice thing to try and catch and, if it shows up at a good time, on a reasonable day and with a non-US registration, that is even better.  This example has an Isle of Man registration.  The Isle of Man is part of the UK but is a tax haven and decided to set up its own aircraft registration system.  I guess it has been quite popular.  This jet came in to Boeing Field following is transatlantic crossing.  I don’t know where it headed next.