@Juve I have the same problem, constant notification of a device disconnected from my Wi-Fi network "Unknown" that does not appear in the list or connection history 

I sent the support an email about that and other things (firewall, connecting to the deco locally through the app when there's no internet and a few suggestions). I hope to get a response back soon and I'll update the relevant answer.

I couldn't find much info about that on the web...

@Juve  This is the response sent by support to the unknown device query

para mí

@Juve The worst device i have tp-link i have no idea why even they made it? now these devices are showing in my router as unknown. and when i am trying to add them into blacklist its saying unknown is already exist , well it will be because i have blocked one unknown device . when i am trying to add other one its saying i can't. and there is no way i can assign a name to device , what a point to have i.p address and mac address when you guys cannot use it,

here is my other problem and their I.T team can not help.

.1 if i have parents rule, and i put device in blacklist they still can access internet. i have to turn off parents rule for that device then blacklist work .. 

i wanted to throw that bad device ...