The GPIO is floating when you do not have any pull-resistor (internally or externally). You will then sample either 0 or 1, based on the noise in your design, on that specific pin.
need to add a pull-up or pull-down register.
You should always have your pins set to a defined level, or set to "disconnected" in the NRF_GPIO->PIN_CNF[] field INPUT.
floating inputs can lead to a undefined voltage level on the input, and thus cause excessive current consumption in sleep.
