^^^^This is accurate. "Tether" is not really the correct term.
Tethering is using your phone like a modem, with your computer connected to it with a cable. Not very common. More common is the practice of using a phone to make a wi-fi hotspot. On my phone you must have physical possession of the phone to do it. The hotspot is secure like any other secure wi-fi---you have to choose which devices are allowed to use it.
If in some unlikely way the phone had been used as a hotspot, by someone else, it would become obvious because the data usage would go out through the roof.
And the problems may have nothing to do with the phone.