That only means that "Jamie's Mac" has file sharing turned on, and is visible on the network that you are using. That does NOT necessarily mean that you can connect to Jamie's Mac, or that Jamie can connect to yours (unless you to allow it.)
What other Macs use the network that you use?
Remember that a single wireless router may be able to provide enough signal to connect 50 to 75 feet away, at the most. Any computers further away might see your router, but not be able to connect at all.
I assume you meant WPA2 - there's no WEP2. And, that's for your wireless signal, which does not affect someone who might connect through ethernet - assuming your router is used for that.
I guess a better question would be - Do you know Jamie? If it is something you have never seen before, it is likely a next door neighbor - and they have file sharing turned on.