There are a few ways around this, they are using.
To answer your question... yes. You can see the packets.
I've lost count of all the ways this stuff happens, but I'll tell you that, at my house, my wife found an unknown device on the network. She accused me of having another cell phone, because the MAC address was that of a cell phone... but she couldn't show me proof, at the time. When I looked, I did not see the packet transmission or the device.
Concerned that my wife actually thought I had a secret phone, I looked into it and discovered the following.
First, your Internet provider automatically subscribes you to a type of customer feedback program. This is the primary vector. There is a virtual modem inside your modem that turns on when it thinks your aren't around and transmits your "customer feedback" data to your ISP. You CAN opt out of this program, but they give you $5 a month discount to have it, so your bill will go up. It still won't help... and I'll explain why.
What is happening, is that all of your Bluetooth devices are "Bluejacked", and they are all communicating with each other whether you have your Bluetooth on, or not. The way Bluetooth communicates, it does not require a handshake to just spill out data, so any device set up properly can receive data from your Bluejacked device.
So, when you are asleep, at like 3am, not only does your ISP open up a portal, your Bluetooth devices also play a little game.
Hidden virtual drives are being created... even your TVs are doing it. When that portal is opened, they upload the data, then disappear.
Not only is it your data, anyone who comes to your house is compromised. Immediately, your friend's device is making connections to you that marketing firms, governments and rogue AIs have access to.
Go ahead and test it. Buy a burner device, use it at home, and see how long it takes to identify who you are. You'll know it's figured you out when it offers you all your contacts as suggestions.
Keep in mind, the virtual modem inside your modem that uploads to your ISP (which goes to "search guide inc", IIRC) is just one of many temporary portals that open up.
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