GNOME keyring stores all the passwords that you create in any apps on your device. So if you set up an email account in Geary, and told Geary to save your user name and password, then GNOME keyring saves it so that you don’t have to enter your email login data every time.
You may have simply used your L5 PIN to save the data in GNOME keyring, and then forgot about it. Or the keyring may be asking you to create a password in order to store the data.
Also, if you install Seahorse, either through the terminal or by using the search window in the Software store app, you can see all the apps you’ve stored passwords for. I’ll link you a useful thread about Seahorse (aka Passwords & Keys): How to change Gnome-Keyring Password? and List of Apps that fit and function well [Post them here.]