Thanks but I wasn't reusing a password.
Deleting the note doesn't resolve anything − the issue is that someone else accessed my account even once.
Upon looking at https://www.evernote.com/AccessHistory.action, I see that no other access has been recorded, besides my own, so this note must've arrived through Evernote's own system, not through the reuse of my credentials.
My phone was always with me, in my pocket or my hands, during the time when this note was created.