Hi phil,
I don’t think that would be your ZigBee network… Do you happen to have a Chromecast or other streaming device on the network? As I discussed in this post here, there are many devices that deploy networks like what you’re seeing in order for them to be casted to, or interacted with in some way. Regardless, you should be able to find out where/what it is by playing hot/cold with the signal strength!
Let me know if this helps!