After playing around with it. I think that what is causing this to happen is that the JetDirect port on the printer (usually 9100) is getting written to by a port scanner. This will cause a printer using JetDirect to print out whatever gets sent to it on that port. Try it yourself if you have a printer that implements it. For me it was a Brother HD-5370DW.
1. telnet <printer> 9100
2. Type a hello world message.
3. Close the connection
4. The printer will print out whatever you typed. At least it did for me.
I sometimes get the same ones at work! It's the crawler from the Baidu-search-engine checking if the printer is a web-server.
I contacted ITS about it (obviously, you shouldn't be able to print from outside the university) but they haven't really given it any work. It surely is a security hole, and a minor waste of ink & paper.
Actually, it's somebody searching for an open proxy, note the inclusion of http and hostname in the GET. The baidu crawler wouldn't be so ridiculous as to request its own homepage from your server. Somebody is testing to see if they can get your server to proxy to baidu for them.
1. telnet <printer> 9100
2. Type a hello world message.
3. Close the connection
4. The printer will print out whatever you typed. At least it did for me.