This is a question I get relatively often, and it’s usually nothing to worry about.
Certainly viruses or spyware are a possibility. Of late I’d also call out “Foistware” – the software that often appears on your system unexpectedly after installing something else.
However there are also more benign scenarios. It’s possible your browser may simply be doing a good job protecting you.
Spyware and Foistware
Spyware often works by forcing your browser to go to places you would never go. Foistware in particular often adds toolbars and other software to your system that intercepts what you’re doing and just rudely takes over, sending you to bogus search sites or worse.
These days my first recommendation when you run into unexpected history entries is to make sure your machine has no unexpected malware. In particular I’d have you run the tools outlined in How do I remove PUPs, foistware, drive-bys, toolbars, and other annoying things I never wanted?
Many websites employ pop ups – additional browser windows that open up when you visit the site. These typically contain ads for which the site owner is getting paid.
Pop up blockers were created to prevent pop up windows from happening. Using different techniques, the blockers intercept the requests to create new browser windows and prevent them. Most modern browsers include pop up blocking technology of some sort.
Pop up blockers insert themselves into the normal processing of web requests, and “kick in” when they detect that a pop up window is being requested. Exactly where depends on the specific blocker and how it operates. It actually gets fairly complicated, since there are so many different ways to create a pop up window. That’s why some blockers are more effective than others.
My theory is simply this: a URL that’s been requested to open in a pop up window may actually get added to your history before the pop up blocker kills it. You’ve never seen it, but it’s there in the list.
One page, multiple sources
History entries certainly don’t have to be created by pop ups alone.
Some pages on the web that look like a single page are, in fact, a composite of more than one page or page fragment. What you see as being a single URL may in fact, have fetched content from several different sites.
Depending on how your browser handles these requests, it’s possible that these sub-pages or page elements could also show up as individual entries in your history. You wouldn’t recognize them, because they’re not pages that you explicitly went to, but they’re present because they were part of some page that you did visit.
Ultimately, unless someone else is using your computer when you’re not looking, I believe that this is typically a very benign occurrence.
However that shouldn’t stop you from taking the appropriate precautions to keep your computer safe.