TL;DR: Warning signs after using public WiFi include being unexpectedly logged out of accounts, a browser warning about an invalid or unexpected security certificate, or account activity alerts for logins from an unfamiliar location shortly after your session. Change passwords for any accounts you logged into on that network, particularly if you weren't using a VPN or the site wasn't using HTTPS, enable two-factor authentication where available, and use a VPN on any public network going forward.
Warning signs after using public WiFi include being unexpectedly logged out of accounts, a browser warning about an invalid or unexpected security certificate, or account activity alerts for logins from an unfamiliar location shortly after your session.
What Usually Causes This
Public WiFi is often unencrypted or uses shared credentials, making it possible for someone else on the same network to intercept unencrypted traffic or set up a fake network designed to capture login details.
What to Do About It
Change passwords for any accounts you logged into on that network, particularly if you weren't using a VPN or the site wasn't using HTTPS, enable two-factor authentication where available, and use a VPN on any public network going forward.
If this looks similar to something else you've seen, it's worth reading How to Remove a Device You Don't Recognize From Your Apple ID.
This pattern shows up elsewhere too — see How to Remove a Device You Don't Recognize From Your Google Account.
Frequently Asked Questions
What are the real warning signs?
Warning signs after using public WiFi include being unexpectedly logged out of accounts, a browser warning about an invalid or unexpected security certificate, or account activity alerts for logins from an unfamiliar location shortly after your session.
What usually causes this?
Public WiFi is often unencrypted or uses shared credentials, making it possible for someone else on the same network to intercept unencrypted traffic or set up a fake network designed to capture login details.
What should I do about it?
Change passwords for any accounts you logged into on that network, particularly if you weren't using a VPN or the site wasn't using HTTPS, enable two-factor authentication where available, and use a VPN on any public network going forward.
Should I be worried if I only see one sign?
One sign alone (like a slower device) is usually not cause for alarm — most single symptoms have mundane explanations. Multiple signs together, especially unexpected data usage or unfamiliar apps, are worth investigating.