If it makes virtually no difference in the other person's life, then I don't think it matters, because it making virtually no difference, by definition, makes it unnecessary information. Does not telling someone despite it possibly making no difference indicate a worrying lack of trust? Possibly, but not unconditionally. It could just be something the person wants to forget, and your partner has no right to that information anymore than they do something as basic as your phone password in that context - but who knows, I'm sure some relationships out there demand for even basic levels of privacy to be dismantled.