PopIn keeps you signed in on each device you use. If you ever want to check where that is, or boot a device you don't recognise, it's all in one place.
See your active sessions
Go to Settings → Security and scroll to Active sessions. Each row shows the browser or device, a rough location, and when it was last active. Your current session is marked so you don't sign yourself out by accident.
Sign out a device
Find the session
Spot the one you don't recognise, an old phone, a library computer, anything that isn't you.
Revoke it
Sign that session out. Whoever's on it is logged out immediately and needs your credentials to get back in. You can also sign out everywhere at once.
Sign it out, then change your password right away (same Security tab), and turn on two-factor authentication if it isn't already on.
Trusted devices and 2FA
If you use two-factor authentication, you can mark a device as trusted so PopIn doesn't ask for your second factor every single time on it. Manage trusted devices from the same Security tab; revoke trust on a device you no longer use.