Scans not detecting all cookies , Virus attack caught by Norton on Facebook, NIS

  Last night, I was on Facebook and I clicked on a friend's link, and a Norton Window stopped the screen, saying that an attack on my computer had been stopped by Norton, included my correct computer name, and a ip number where the attack came from. I logged off of facebook.  Today, when I do a scan ,some cookies are detected and removed, others say name of cookie

and underneath undetected.. What would cause this?