I have not heard about the ancestor coming to visit as moth/butterfly tale among the chinese before.... perhaps it is only believed by a certain ethnic group of the chinese?
I was aware that moths do visit funerals quite a lot but not as the souls of the deceased... infact, there's a research that was done to investigate why do moths frequent these situations all the time. According to the research, they found that the ritual items, color and among many other things, tend to attract moths to the funeral parlour.
However, I have heard that the deceased 'soul' do wonder back to the people they're close to. It has happened to me personally, and also to my cousin... that was about 20 years ago when my grandfather passed away. I think it is logical and nothing strange for the deceased soul to wonder around as we all know that it takes a certain period of time for them to move on to the next life... the interim between lives that is called the Bardo.
But then again, who know... perhaps there is some truth to this old wives tale...