OSX Yosemite network-popdown suddenly have a permanent Device entry
What's that "Device" entry?

The entries under Device are wireless ad hoc networks, a type of wireless network that doesn't involve an access point or router, that is, a point-to-point wireless network, which usually connects two computers or a printer to a computer.

I couldn't find any Knowledge Base article at support.apple.com but theMacObserver has an excellent article about it.

What's that SETUP node? - I guess it's something running on my Mac?

In your case, the ad hoc network is probably a printer nearby that is broadcasting its name. Another usual ad hoc network name is hpsetup, the default ad hoc network used by HP printers.

How do I best scan the SETUP node, to learn more about it?

You could connect to it and run /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -s to get some information, but I don't think that's going to be of much use. You better go over to your neighbor's house and ask whether they have bought a printer lately.

How can I create an ad hoc network?

You can easily create an ad hoc network yourself:


In the Wi-Fi menu in the menu bar choose Create Network...:


Choose a name (that's the name that will appear under Device) and press Create:


This is the new entry:



If you select Disconnect from &lt;network&gt; the ad hoc network is torn down.