The phrase "small footnote" refers to a minor or unimportant detail or piece of information. It is something that is not very significant or noteworthy in comparison to the main topic or main idea being discussed. Full definition
I like that this answer presents factual information first, then adds speculation as a small footnote. (politics.stackexchange.com)
One small footnote: Vul notes that his second postdoc supported by the CPS grant has taken a faculty position in India because U.S. visa rules require him to go back to his home country for a few years before trying to settle permanently in the United States. (sciencemag.org)
Although rejection is always painful, remaining open and positive about the future ensures that the experience can be a small footnote in a long and successful relationship history. (eharmony.com)