When the mail comesI know. Not only do I know when it has come but I know when it is coming and after the mail lady left. And then I know when the neighbors across the street get their mail as well.
I have a Yorkipoo (yorkshire terrior/poodle) and he insists on letting us all know when the mail has come. He is better than any doorbell but more annoying than any device thought of. I never have to wonder if/when the mail has come, Yorkipoo lets me know!!