Does the word "flash" in this context have a same meaning as "rage"?
Actually, this expression is very similar to the more commonly used expression "to have a glint in one's eye," which means a facial expression that indicates a hidden emotion, motive, or agenda. The old man likely has negative feelings about taking care of his granddaughter, with whom he is unfamiliar, so his facial expression ("flash in his eye") reflects those subtle emotions. Ex: The thief looked at the man's watch with a subtle glint in his eye. Ex: He congratulated me on my promotion, but I could tell from the glint in his eye that he was unhappy about it.
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