I think @xfpd is suggesting that if the signal lines from the processor to the sensors are still connected, current can flow through those lines even if their ground pins are disconnected. When you disconnect the ground pin of a device, all of the other pins will float up to Vcc voltage. So if you connect them to GPIO pins on the processor that are LOW, current will flow. As Jeff Goldblum would say, "Ground always finds a way."
Or perhaps he meant something else.