OP here: for clarification I have "email addresses" which are <whatever>@fastmail.com (or whatever other domains fastmail offers). I then use the word "alias" for the on-the-fly alias such as trash@whatever.fastmail.com where the real email address is whatever@fastmail.com. To be clear, the email I received in German was sent to the whatever@fastmail.com (an actual email address, not an on the fly alias). The connection to Germany is that the email seems legitimate to some degree (it was to verify the email address) but it was in German and received within a week after visiting Germany. The email address was not used while in Germany and has never been put into a web form.