Traceroute should nail down the ISP and possibly enough info for the ISP to track it down, but you need law enforcement to get further I guess. Personally, I'd rather have full disk encryption and password login to prevent them even getting far enough for a network connection to be there.

I'd use the Core Location api on OS X and probably use something like https://github.com/victor/whereami (no need to invent the wheel again). When connected to a wifi, it's rather simple.