Power MOSFET (Smart HITFET) back-feeding and turning itself on
The data-sheet / appnotes say nothing about this behaviour.

The data sheets tells you that the on-state input current might be as high as 30 uA. It also tells you that the input threshold voltage for activation might be as low as 0.8 volts (worst case). So, I would naturally want to choose a parallel resistor that was lower than 26.7 kohm. I'd use a 10 kohm pull-down resistor. In the absence of any other information you have to make of the data sheet what you can.
If in doubt, consult the manufacturer.