my wifi is connecting to neighbors wifi. Could it be a child of mine is connecting an unknown device to have full access to internet?

I'm not sure whats going on but I noticed a teen of mine being pretty creative and getting around parental control when I discovered he bought a iPod we did not know about. This meant that his iphone had supervision and was left to charge in family room but once in his bedroom, he had free reign to the internet world without supervision with his secret purchase. I outsmarted him by changing our families wifi password. So, Now I am noticing that when I come to my desktop I Mac, our wifi is all of a sudden changed to a neighbors guest wifi with no wifi password. I then switch the computer over to our wifi. What is going on? Is it possible that he could be using a device in his room and connect to neighbors and the connection shows up on my computer when switched? I have never noticed this before now. Since I changed my wifi password. Ive asked the neighbor to add a password and she said she will. Id love some insight on this.