I've been running some custom software for sniffing probe requests from my apartment (my log shows over 30k unique devices). The most interesting one I've seen is Teslas... They'll probe for "Tesla Service", and sometimes other networks (perhaps the owner's home?) as well.
In addition to probing for Tesla wifi networks (found at service centers and showrooms), you can configure the car to use additional wifi networks, like your home and work networks. The car's always on 3G; the main purpose of wifi seems to be lowering Tesla's data bill for sending out over-the-air updates.