On Pi 3 with Raspbian (clean install) when I run:
sudo iwlist scan | grep ESSID
I can get both normal outputs and outputs with empty ESSID. E.g.:
ESSID: "SomeNetwork"
ESSID: ""
The list does not contain some networks which I can see through the web interface on my laptop, so I guess they are shown as having empty name.
Does anyone have any idea what is going on here?