[uk_dave&gill]

I think this article probably answers the question, specifically

Windows File Explorer > Network > displays all devices on a network. So the device is 'on' the network (your router with extender), but not *IN* your network. Any device can try to 'access' the network by first 'listening' for wireless signals like a radio listens for radios stations. In this case the phone (a neighbor) has WIFI turned 'ON' so it automatically looks for WIFI instead of using cellular minutes. Your wireless and extender are broadcasting "BOBSWIFI", so the phone tries to automatically tune in BOBSWIFI, but then when the phone and wifi/extender talk, your network says "PASSWORD!" and the phone goes "Huh?" and that is it. So it doesn't get 'IN' your network.

Now if you don't want any device to 'pick up' your broadcast, you would need to turn off BROADCAST NAME option in your wireless, then MANUALLY add the wireless to your computer, iphones, etc. by putting in the name, password, etc