web server: how does this request appear

I'm building a web server with Python-tornado. The server can simply process get requests.

I also make a simple log.

In the log, I can see that most of requests are like this:

[I 170625 19:23:12 web:2063] 200 GET /images/icon-language.png (116.31.83.132) 0.88ms
[I 170625 19:23:12 web:2063] 200 GET /index?type=Sight&key=Bol%20content (116.31.83.132) 10.05ms
[I 170625 19:30:30 web:2063] 304 GET / (116.31.83.132) 0.87ms
[I 170625 19:30:44 web:2063] 200 GET / (116.31.83.132) 0.78ms
[W 170625 19:30:51 web:2063] 405 POST / (116.31.83.132) 1.20ms
[W 170625 19:31:00 web:2063] 405 POST / (116.31.83.132) 0.63ms
[I 170625 19:31:22 web:2063] 200 POST /index (116.31.83.132) 0.89ms
[I 170625 19:31:42 web:2063] 200 GET /index (116.31.83.132) 0.62ms
[I 170625 19:31:49 web:2063] 200 GET / (116.31.83.132) 0.78ms
[W 170625 19:31:57 web:2063] 404 GET /abce (116.31.83.132) 0.65ms

But to my surprise, there are a few of requests as below:

[W 170625 18:43:41 web:2063] 404 GET http://baidu.com/ (106.2.125.215) 0.60ms

I can't understand how this kind of request is generated. For example, if the address of my web server is www.example.com and I send some get request to it, it must be like this: www.example.com/abcd. But this request doesn't start with /, how comes?