Edited: I have now Bluetooth hardware disabled and a different phone appeared today in "network" folder.
Looks like I was wrong and it isn't Bluetooth.
Hi,
I have had the very same issue since I've upgraded to Windows 10 Pro.
A stranger's phone occasionally appears in "Network" folder under "Phones". The name of the device not always the same. It sometimes comes and then goes.
I have contacted Microsoft support about it and they have connected to my PC through remote control and ran several checks to find the issue. But they could not find the cause.
However I got closer to the answer. Them devices are Bluetooth devices. If you go windows settings and disable Bluetooth nothing will change. Them devices will show up again and again. But if you go to device manager and disable the Bluetooth hardware then you will never have them strange devices showing up in "network" folder.
This is something really strange. I have contacted Microsoft support today again and told them about it.
They have run multiple checks again and couldn't find a thing again. They have seen the stranger's phone appearing in my "network" folder but couldn't tell why.
At the end of the chat they have said they going to investigate it further and thanked for the information I have shared with them.
At the bottom line. Just disable Bluetooth hardware in device manager.
Before anybody asks.. Yes I have scanned for malware and viruses. I have real time security application.
Kaspersky Total Security. I have a Virgin Media broadband with their "Superhub 2ac" and yes I have a password on my wifi. WPS is disabled. I have checked all logs possible and found NO unknown activity on LAN (or wireless).
Hope it helps some!

I have been concerned about my systems security since upgrading to window 8 last November on occations a phone has appeared in my network devices list well windows knows the phone it can get make model and mac code but after clicking on the phones propities
it disappears. At first I thought it might have got in through my wireless router which is a virgin super hub made by netgear and after a little digging I changed the password and even the way devices connect wirelessly and made my hug a mac code only connection.
This worked for a few weeks then hey presto another phone or tablet appeared on my network. I even paid a extra few quid to Mcafee and upgraded to access all areas to cover my tablet and my our phone the issue persisted . I was so annoyed at this time but I
had noticed the my processor was a 64 bit capable processor so I took the plunge and brought another windows pro 8 pack as the system had to have a clean install. All has been fine for six weeks since I upgraded to a 64 bit machine and suddenly it has all
started again I have had three different devices appearing in my network map and I have not got a clue if my system has been compromised. Or is it this think is a way for others to connect to the internet for free using my isp which is illegal.
Does anyone have any ideas where I turn to, to solve this problem ?