Beware of a man with a gleam in his eye

my daughter has been worried after buying a fortune cookie, could you explain what this means. Beware of a man with a gleam in his eye,it may just be the sun shining through the hole in his head.

The only gleam I know about is romance/lust. As in: you were just a gleam in your father's eye. Meaning your father's interest in your mother that lead to them becoming parents.

I think there may be another kind of gleam, the gleam in the eye of a man on a mission, or a man with an obsession, a man with a plan. It is easier for me to imagine such a man as having a hole in his head than one whose gleam represents merely concupiscence.
SS