socket.io on（'connection'）触发没有实际连接

Question

var http = require('http');
var express = require('express');
var app = express();

var server = http.createServer(app);
server.listen(3000);

var io = require('socket.io').listen(server);
io.set('log level', 1);
io.sockets.on('connection', function(socket) {
  console.log('connected');
});

运行后

node server.js

有时“已连接”会记录到控制台而没有实际连接，它来自何处？