Master and Commander recurring dots

Numerous times during "Master and Commander", I noticed a grid of dots
appearing briefly in different areas of the screen.  At first I thought I was
seeing dirt, but it occured so often that I finally realized that they must
have been some sort of special effects artifact.  The dots appear in what
looked like a 3x3 grid, but there were not always 9 dots, sometimes 3 across
the top and 3 down the right side.  They reminded me of Braille, and were most
noticeable when they appeared against the sky or some other plain background. 
The appear and disappear so quickly, they couldn't be present for more than two
frames.  Has anyone else noticed these?  Anyone have an idea as to what they
are?