Could there be a closer star to Earth than the Alpha Centauri triple star system, excluding the Sun? Do we know for certain that the nearest star to Earth, excluding the Sun and the theorized Nemesis, is the Alpha Centauri multiple star system, at +/-4.4 light-years away? Have we been able to identify every star that is &lt;5 light-years from Earth or is there a possibilty a faint star could be nearer?  Are there stars that we haven't looked at yet that could be closer?
It's begining to look like there aren't as many brown dwarfs as was once thought: 


  NASA's Wide-field Infrared Survey Explorer, or WISE, has been turning up a new crowd of stars close to home: 
  
  ...
  
  Previous estimates had predicted as many brown dwarfs as typical
  stars, but the new initial tally from WISE shows just one brown dwarf
  for every six stars. It's the cosmic equivalent to finally being able
  to see down a mysterious, gated block and finding only a few homes.


Reasonably current data puts the current closest brown dwarf at 6.5LY (2 parsec) and less than 0.08 solar masses.

A nearer one could show up, but it's not looking likely.