I add more info: the red arrow indicates are strangers'.

these days that red arrow indicates are NO ONE at home.

@XJRHD 

By any chance, do you have Guest network enabled? By default, it is without password. I can't tell from your screenshot if these were connected to Main or to Guest WiFi.

Also, there is such thing as "MAC address randomization" which makes same device appear as different every time it connects to WiFi. Google it. Perhaps, when you weren't home you had one of smartphones (tablets) left at home turned on and it comes with MAC randomization. 

If nothing like that, just blacklist these unknown devices in Deco app, and see if anyone at your household complains their smartphone/tablet stopped working. Unfortunately, this won't help for device with MAC address randomization turned on, but perhaps you'll be lucky and it were off on these unknown devices.

@Alexandre. 

Thank for your reply.

Here are my responses:

1. Guest mode is disabled.

2. I had chaned password once and still see others. Then I asked your help.

3. Now all deco are working on AP mode, so does deco still have Main or Child?

4.  As my question, is there any super password that can be used to connect any deco?

I had block these phones when I discovered.

But it comes one by one.

And these seems come from same neighbor.

Merry Christmas!

One more,

All devices such as phone/tablet/desktop/laptop etc are under my control. It's not possible to have device connected which is mine but I don't know.

@XJRHD 

In Access Point mode both Main and Guest WiFi networks are available. Double check in Deco app that Guest network is not turned on, it could happen accidentally and, occasionally, changing settings in Main network could trigger Deco app enable Guest network (yes, it's a bug).

The probability of TP-Link allowing such sort of backdoor you are asking, into their WiFi mesh, is very low. While such things do happen, nobody heard about that in regard to Deco. The probability of your neighbor be the first to find that backdoor (if it exists) and use it for getting to your WiFi network is extremely low.

You need to consider cases that have higher probability. A person who has physical access to device connected by WiFi can retrieve WiFi password from it in clear text. I would look at teenage members of your household and if they have friends of similar age at one of close to you neighbors. That is where password leak could have happened.

You can run tests to see if WiFi password leak happens from your household. What I would suggest:

1. If everyone in your household leaves premises for days, like on your chart, next time it happens - after everyone left, use Deco app on your smartphone to change Main WiFi password and tell it to nobody. You can do that with Deco app connected to cell network. Do not update WiFi password on devices, you won't need it till people are back in the house. Also, after password change do "restart all" for all Decos in Deco app.

See if someone will get to your network without knowing the new password, while everyone is out.

2. Enable Guest network, with different from Main network SSID and password, connect every household device to Guest network. Then, keep Main network SSID but change password on Main network and tell it to nobody, also do not use it on any devices at your household including yours.

Wait till unknown device connects and see if it connects to Guest network. If it does connect to Guest WiFi, that means someone in your household shared Guest SSID and password they retrieved from one of household devices.

----------------

You can also make it less convenient for your neighbors to use your WiFi network: turn off 2.4GHz which will dramatically reduce your WiFi home network range, also turn off all Decos when everyone leaves your house for days so that your home WiFi is off when your household does not need it.

  @Alexandre. 

Thank you for your reply. Here is my comment:

1. Neither kids nor the teenager are existed in the house.

2. Guest network is disabled from the beginning. Never will be enabled.

3. As my former description, I never shared password to anyone. The all phones/IPTV/tablets are under my management.

Now let me update the situation I had now:

1. Those strangers are blocked into my blacklist.

2. Then I changed the password, it seems that no stranger be seen until... let's go to next description : 3

3. On Feb 18, I reset one of my deco X20, then a new phone enters.... its name is android-xxxxxxxxxxx.

    Please be noted that the deco system has set by new password already....

4. Again, this android-xxxxx is not mine.

5. Then it goes to the blacklist.

Till now, I didn't see any new device.

In fact, I've already worried about the security that Deco has. (AP mode)

You may wonder my password 'may' share to others.

But, second time, I did change my password and be more careful to sure  that 'I DID NOT SHARE NEW PASSWORD TO ANYONE'.

Then such un-desirable situation happens again.

I do very upset about this and regret to purchase Deco.

  @David-TP 

Thank for your reply. 

It's difficult to set 'deco' mode as the layout of my environment is not good for it.

My house is 4 floors and the deco in each room is hardly to be connected due to concrete wall.

Each deco must be allocated at the place that it can be scanned and connected by its root node. (wireless)

I was thought that each deco can be connected by wire (RJ45) and support wireless as well.

Anyway, let me think about it.

In terms of random MAC, as I know this happens in iOS but android.

As my former description, the device name is 'android-xxxxxx'.

I think this may be not the cause that I had.