Extra entry in connection strings section of App.config

I've used simple code to iterate thru all connection strings in App.config file:

var connStrings = ConfigurationManager.ConnectionStrings;
int i = 0;
foreach (var connString in connStrings)
{
    i++;
    var connStrSet = (ConnectionStringSettings)connString;
    Console.WriteLine(i.ToString() + ". " + connStrSet.Name);
    Console.WriteLine("\t" + connStrSet.ConnectionString);
}

But I wondered when I saw extra connection string entry which did not exist in App.config:

1. LocalSqlServer
        data source=.\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|aspnetdb.mdf;User Instance=true

Where did it come from? :)

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