"Weird websites" could mean a lot of things. Keep in mind that Squid will report every HTTP access, not just items you type into a browser address bar. Almost certainly you will see sites in the list you had no idea you were visiting through the sites you are intending to visit. The ad people are responsbile for a lot of "weird websites," for example.
As for Squid displaying the public IP instead of the machine's localhost address or a user IP, that I can't help you with. I've never seen Squid behave that way in a Zentyal installation (and Sarg, of course, is simply displaying the content of the proxy's logs). If the machine is not visibile to the public and/or the proxy port is closed/not open to the public, it's almost certain that you aren't running a public proxy. To look at it another way, if the machine is visible to the pulbic you may have more serious concerns than an exposed HTTP proxy.