There are many possibilities. It could be that she has a secret hobby, like collecting rare stamps or old coins. Or it could be related to her family. There might be a family secret that she is privy to and has been guarding. It could also be about an act of kindness she did anonymously. She might have helped someone in a big way without wanting any recognition, and this is her little - known secret.
Perhaps her secret story involves a relationship. Maybe she had a childhood sweetheart and there were some really special and secret moments between them that she has never shared. Another possibility is that it's about a personal transformation. She might have overcome a great fear or a bad habit in a very private and unspoken way.
It could include a hidden talent. For example, she might be an amazing painter but has never shown her works publicly. Or it could be a long - held dream that she has kept to herself, like traveling around the world alone but has been unable to do so for various reasons.