In Oz the national broadband carrier broadcasts a separate network called "Telstra Air" and for international users "Fon" from YOUR private Internet router. Essentially someone can jump onto your Wifi and pay a fee to use your connection if they are situated near your house - they claim it it totally independent and only uses 1-2 mb/s - psshh.
So we have networks being piggy backed on networks.
https://fon.com/faq/ Fon is global and the "dual" network concept exists in many countries.
The tech is right there in your face. Clearly the infrastructure is already built. God only knows what other networks are currently in place. I can almost bet (a large amount of money if I was rich) that all this network infrastructure is being used via bandwidth and tech we as mere private consumers have no knowledge of.
So somebody else's traffic can go through your router and you have no idea who it is (could be a dark web CP deviant - and guess where the IP gets traced back to) - that is fucked and a major security breach that consumers should be made more aware of (it is prob in the fine print of the contract).