The device looks like a FM antenna; there were also some 3G/LTE routers that look almost like that (now they're mostly soapbar or cube shaped, it seems). It has happened to me to be connected to a WiFi and a cell network, or run diagnostics on one while connected to the other. And fiddling with a cell antenna's position in the hope of getting better reception/bandwidth is a hallowed tradition.
That said, could have it been a hacking device? Yes. It's remotely possible. Not a WiFi though, for he would not have needed it any closer than several dozen yards, and not Bluetooth either (a couple of meters; much more with such a large antenna. "Cantennas" and "gun assemblies" have been reported allowing Bluetooth phreaking at hundreds of meters). NFC isn't a likely contender with a laptop. All the above, assuming we're not dealing with a moron -- of which there are many more than there are hackers, and whose M.O. is closer to stalking a Starbucks than a hacker's would be).
Was the donut shaped part upright, or parallel to the table? If the former, it could have been a short-range EM receiver possibly suitable for a Van Eck attack (also called TEMPEST attack). At short range it is possible to passively eavesdrop on LCD monitors and keyboards (sort of remote keylogging), and even CPUs. While the device described on Wikipedia had a range comparable to WiFi, it also costed in the neighbourhood of USD 2,000; a cheaper device with a range of less than one meter can be built with a tenth the expense.