From the GroundBased post of June 13, 2017:

Mackey Chandler’s short story Adverse Possession deals with a confrontation between an official ship from Earth reaching Mars and finding, much to the consternation of all, that this particular celestial body is already inhabited.  The story is set in the near future, close enough that readers who are space lawyers may feel compelled to assess the situation in light of current law.  SPOILER ALERT (this is where you pause and go read the charming and entertaining story–it’s a fun, quick read.  I’ll wait):  What threw me was that Mars was not inhabited by Heinlein’s fundamentally scary Martians, or by Edgar Rice Burrough’s Deja Thoris and Tars Tarkas, but by people from Earth.

How the heck did people of Earth get to Mars with no one knowing about them?  Under current law space flight participants have to sign reciprocal waivers of claims with the U.S. Government and, as of 2015, with the launch operator.  The launch operator has to get an FAA license to get off Earth, and that process involves saying where you are going.   (Sure, these laws may not survive the future, but laws seem hard to repeal, even those that are designed to sunset.  How many times have the so-called “indemnification” provisions of the Commercial Space Launch Act been extended?  A lot.)  It seems that someone at the FAA would know that people had gone to Mars and who they were, and might mention it in passing during an inter-agency meeting or three.  We know for sure the agency would have issued a press release.  Had the secret travelers availed themselves of the regulatory provisions protecting proprietary information?   Even though I have said SPOILER ALERT I will not give away the answer.  I will only say it’s not legal.