"Accidental" release of modified version

[Richard Stallman <rms at gnu dot org>]

I showed this to our lawyer:

  However I have a concern that this creates a loophole -- not a legal
  loophole, an illegal loophole (AFAIK), but a loophole nevertheless. 
  Let's say I create such a derivative work, and I don't GPL it.  But I
  leave it somewhere (accidentally, with no malice aforethought) where
  somebody else finds it, with no copyright or GPL notice.  Now they take
  it and use it as is or create a derivative work, and distribute it as a
  closed source program.
  
  What is the status in a case like that?

He responded:

No [this is not a real issue].  This is the same, functionally, as
someone who takes GPL software, modifies it, releases under GPL to
comply with 2(b), but doesn't enforce the license when someone else
violates it.  No loophole has been created, because authors of prior
versions, from which this was a derivative, can enforce.  The modifier
who "left lying around" was making non-compliant distribution, of
course, and all previous authors and assignees can enforce against
him.  All subsequent takers of the software can only redistribute
under a license, and the only license available is GPL.