As I understand it, it stays on mission until told to do different.
As for the S1 switch, if it's able to receive signal from your RC, then any movement of the S1 switch may take it out of GS mode.
It happened to me while the Phantom was about 100ft away. I accidentally bumped the S1 switch, half way through its mission. It discontinued the mission and went into Atti mode, floating away with the breeze. It then traveled about 200ft away before I realized what was happening and retook control.
Try it. During a mission, flick the switch once, not both ways. Tell us what happens. I would do it, but my Phantom is in China being fixed.