The P2 was shipped to you in the Phantom mode and if it's still in that mode in the software there is no ATTI on switch S1. In the user's manualv1.4 on page 25 & 28 there is an explanation on how S1 works. If you have already set the third position on S1 to failsafe in the P2 software do so and set the return to home (RTH) altitude limit to at least 120 feet. The second position on S! may say ATTI but ti's no more than an extension of GPS. http://download.dji-innovations.com/downloads/phantom_2/en/PHANTOM2_User_Manual_v1.4_en.pdf
To answer you question on flyaways. Flying in ATTI will not prevent a flyaway .