Student or Learner
What does it mean when somebody's eyes flash?
For example: His eyes flashed and he said: You know what the hell I mean. Stop pretending.
I take it to mean he looked angrily at the guy.
Of course, eyes do not flash except when they may reflect a flashing light, although sometimes eyes are described as sparkling.
You're right in this case - it means the person opened his eyes very wide, thereby betraying his anger.
Uncle M (not a teacher)