I have many times heard the term "roving eye" used to mean a tendency to look around for possible conquests, particularly when the looker is already in a relationship. I have heard "wandering eye" used only very rarely with this meaning. However, when I was young I knew someone who had a medical condition which caused one of their eyes to not direct itself properly towards wherever the person was looking. He was often said to have a "wandering eye" and I believe this is still an informal term for this condition.
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