Look like thunder flies to me...
http://en.wikipedia.org/wiki/Thrips
I had some work their way into an old PC LCD, they're attracted to light.
you must check the enclosure. Watch the lamination of the glass and aluminium body maybe there is a spot where can little things like bugs or dust can get in.
I heard and saw 2012 imacs with the screen detached about 2-3 millimeters from the aluminium back