bjoyce
The cache on your server is a memory of sites and pages visited by your users.
So at sometime your users requested the sunset search and got the porn image due to a incorrect search engine result, or in fact your users did surf to the porn image at some time.
It's probably just a wrong search engine result which is still in your cache, or perhaps there is external hijacking of the site going on.
I don't really think any of us can give you a definitive answer of what your system is doing.