I have found an Xbee3 plugged into my car's usb port to be the most secure as it is 100% local and only powered when the car is running. It also has much better range than the SmartThings arrival sensors.
You can also take the battery from the SmartThings arrival sensor and cut an end off a usb cable and solder the usb wires to the battery contacts of the SmartThings arrival sensor and use the same as an Xbee3 but just not as much range.
Added safety feature is it will open the garage doors whenever the car starts to prevent carbon monoxide poisoning. If the kids get a hold of the car's remote starter or get into the garage and the keys are within range and get the car started or if a bedroom is above the garage (Should be sealed but...)
It only takes one time.