Re: I thought LAN cables were shielded
This was my first thought. Take my building. We have, just in this building, over 200 PCs, various MFDs, several Wifi access points, hundreds of IP phones and various other devices (such as cameras, door swipe card readers and network switches) hooked up via ethernet. Most of these devices are left on, or in standby, 24/7.
The structure of the building itself makes radio transmission/reception difficult (hence we have a need for a lot of Wifi access points). This isn't by design. the building is hundreds of years old, so radio wasn't invented when the building was designed.
But assuming you can get a strong enough signal to read, you are going to need some processing power to sort out the thousands of signals to get the one you want. Assuming you have enough fast storage, you could dump the data to it, then process it off line, but even that's a hell of a lot of effort (and expense) for something that may not yield any useful data.
Even under a best case scenario, you would probably need physical access to at least the building, and if you have that, there are a number of far quicker ways you can get access to the data on the network, even if it's just hooking up a single board computer somewhere in a rack with a suitably large SD, using that to sniff the network links and sending the data out wirelessly in bursts to someone elsewhere with a laptop connected to the SBC via Wifi. Most companies, a suitably talented person could just go in and change the SBC's storage device every few days. No one would think the question someone who appears to be a cleaner or security guard checking the room.