[notriddle]

> How about popping up a message

Doesn't work. Between the cookie pop-ups, update notifications, and "you've got mail"s, people have learned to ignore pop-up notifications.

... of course, the real problem here started with "Bob checked his email on Alice's computer." There are so many ways it can go wrong, like Alice using a browser other than Chrome, Bob using an email service other than GMail, or Alice deliberately installing a keylogger on that computer...

[amaccuish]

You mean like the popup of Google asking if I'd like to sync, or if I want it set as the default browser? It seems to be ok when it's in Google's favour.