It's not quite the same thing, I don't *think* (I'm quite a noob to all this stuff, but trying lol). This is an app called Tplink tether which you have to log into to do things like reset the router, look at data logs, things like that. I think you're referring to the connect to wifi buttons which show all available connections, like neighbours etc. This isn't that.
Let's try to "un-noob" you.
First, it is kinda sorta all the same thing. First you have to understand that your "router" is actually much more, and the part we are talking about it's not really a router at all, it's an access point. These all-in-one appliances that people generally refer to as routers are routers, switches, firewalls and access points. And, sometimes modems. We'll focus on the access point part. That's the broadcast and receiving radios that connect other radios to your network using approved standard radio frequencies and encoding protocols (also called "WiFi", which is just a made up marketing term for all that stuff I just said.)
There are multiple types of WiFi networks, but most people will only think about the 2.4GHz and 5GHz frequencies used in residential type access points. They can be configured to be open or secure and visible or hidden. A visible network will broadcast an identifier known as an SSID. A hidden network will not broadcast the identifier so you must know it to connect. An open network will allow anyone within range to connect to that subnet while a secure network will require authentication first with the higher levels of encryption providing greater security.
Okay, if you're still awake ... Assuming you have your wireless network secured in some fashion, when you connect your phone, you see a list of all available networks' SSIDs being broadcast. You choose yours and enter the password (also called a passphrase). Now your TP-link app can connect to your router because your phone is connected to the network it's serving. And because TP-Link made it to be discovered by the app, it found it and used the name. I would also guess that TP-Link had you register when you first set everything up. This let's you connect to a central server and contact your router should the access point fail or your phone is out of range.
Routers are not the only things that broadcast SSID's. Printers can and so can phones, when they are configured to be hotspots. That's what I think happened to you. Since you said the device disappeared after someone left, it most likely their device was either configured to broadcast an SSID as a hotspot, or setup to act as a modem to allow other devices to access the internet through the mobile network. The app simply saw something (unknown device) and was asking you if you wanted to connect to it. Another possibility was that your apps saw your own router as another device, if it had a guest network initialized and it was letting you know there was traffic through it.
But this morning, there under the normal router, was another Tplink one, saying unknown device, I've posted a pic above. it disappeared when someone from the family drove off this morning, so I'm wondering if they've got another router and are trying to circumvent my controls, but then why not just use a VPN? None are blocked.
If you gave them only access through a guest connection, it will allow them to access the internet but not your internal network. If they connected to your wireless network by you giving them the password, then there's no need to bypass anything. They had access. If you had neither a guest network nor gave them credentials, then they didn't have access and what you were seeing was simply whatever device they had broadcasting an SSID. Nothing more.
It's very important to me because for my job I need to ensure that I'm squeaky clean, I can't afford to have internet traffic to my name which is anything illegal or dodgy. It's stressful.
A good rule of thumb is to not allow anyone you suspect capable of dodgy or illegal activity access to anything, let alone your network, even if it's a family member.
Now, I would point out that the app, unless was specifically listed as a feature of the device allowing you full access, is merely a convenance. Most, if not all routers, switches and firewalls have logging capabilities and allow you access if you log directly into them. As I said before, i have TP-link access points and they will let me monitor traffic in a very granular way as would netgear or any other manufacturer. You just have to learn the method to do it. They sell the app because it's easiest for most people, but this also means that in all likelihood, you are aggregating data on their servers to do it.
