Whenever you log in to Space, it creates a user session for you for some specific time period which may vary from a few hours up to several weeks (if your have the Remember me option checked). Untill the session expires, you will stay logged in even if you leave and come back later.
The Authentication Sessions page lets you track the history of logins and spot an unauthorzed access, as you can see when, from what device and IP address each session was started.
To view authentication sessions, on the personal navigation sidebar, choose Security and go to the Sessions tab:
You can also terminate (revoke) expired or long inactive sessions in bulk by clicking the Clean up button and choosing the desired options.