At times without our realising someone else can open our app in another device and as and when this happens you get a message in the original device that the XYZ app is experiencing a New login and the message can be as below. Dear Name, we detected a login into your account from a new device on a Date at 05:40:21 UTC.
If this wasn't you, you can terminate that session in Settings > Devices (or Privacy & Security > Active Sessions).
If you think that somebody logged in to your account against your will, you can enable Two-Step Verification in Privacy and Security settings.