It could very well be little air bubbles that you exposed. Once they're open, they get filled with epoxy dust from the sanding. I used an electric toothbrush wet with rubbing alcohol to remove the dust from the tiny holes. It was the only thing I tried that worked.
You could sand a little more and maybe you'd sand away the pinholes entirely, but it's just as likely you'll uncover more air pinholes.

: It's most likely glass you're hitting, pretty easy to do when sanding,
: specially if the glass wasn't perfectly flat ahead of time.

: Bill H.