Looking at ActivityMonitor, I saw a strange process with a URL for a name:
I went to that site, and yes it looks familiar I might have been there, once, a long time ago. Definitely not at the time that this process showed up, and even if I did - why would visiting a website trigger its own custom process?
Whenever safari loads a tab a new container is created with the name of URL showing in the process list that's the intended behaviour, and the container goes away when the last web worker belonging to that container terminates.
If the tab is gone what your likely experiencing first is a unintended side effect of a naive standard conform implementation of web workers API where a tread is essentially independent of the DOM tree inside the tab and will continue until it exits or the browser session is closed, unless the browser is aware of orphaned web workers and kills them.
OSX compounds this problem by not exiting sessions when the last safari window is closed, meaning that for most users the sessions last until reboot.
