Well, if the receiver looses the signal from the Tx, any noise, leftover signal or random rf will drive the servos. If you have a PCM system, the failsafe will kick in and move/hold the servos iaw your setup in the Transmitter. With a PPM (also known as FM) the movement will be random but most of the time the heli will head toward the most expensive item around the flying field - or your head. So don't turn off the Tx in flight. You should always turn the transmitter on first and off last. If you run out of battery power while flying you deserve to loose your helicopter. It won't go very far, all helis are unstable. Most Tx have a voltage readout so you can keep track of the battery condition.

Fritz