Distortion and lines around cursor.

I am a technician with a school district and I've had a few teachers bringing me MBPs and one Air that have this strange series of thin black lines that follow the cursor around, they're kind of "boxing" it in, no other screen distortion and it only happens once in a great while.

I checked their cabling and it hadn't came loose, diagnostic checks said display was fine.

In fact, even if the distortion/lines are happening, when I switch to a different screen or reboot it or something, the lines disappear. I don't think it's the actual display because it only shows it intermittently and only one screen, but not the next, etc. It isn't program specific, either.

Any thoughts?

Graphics card, display, software related?