Google analytics (time on site)

I’m currently using google analytics to display visitor reports from my site, and personally I think it’s better than webalizer and other competitors.

However, I have a question that I was hoping that someone would be able to answer:

We get some visits from the US (we’re in europe) that last 00:00:00 seconds which is, of course a bit odd. I’m guessing that:

a. Analytics can not tell how long the visitor has actually been there (for some reason)
b. It’s some sort of server ping request and not an actual visitor

Is any of the above true? And if so, how come?