I have a similar but different happenstance - I ended up writing backstories to sideline characters. Some of them might end up just the story itself later on. For example, Mc had a father who was in the military and fought in WWII. It's related because the father became involved in the story as a “wise old man”. So I ended up with notes about his previous experience during and after the war, the list of his teammates and roles they were filling up, who married, who got widowed. Crazy.