I am fairly new to quadcopters and due to an ESC issue I have not even managed to run my battery flat (until today) I was flying in GPS mode, compass was calibrated, and waiting for IOC lock and GPS lock/home point - all fine. I started with 3/4 battery and wanted to fly it around to run the battery down to see how long it would last for and to practise a bit of flying. I checked after about 15 minutes and the battery was still showing 2/4 bars. I flew it for another 5 minutes or so and suddenly it shot upwards and started flashing red lights instead of green, and started making off in a very approximate direction of the home point (not convinced of this because if it had landed on that course it would have been 20 or 30 meters away). It was not responding to RC inputs so I switch to ATTI and brought it down (at least I think it was me bringing it down). When I got to it the battery was showing 1/4 bars. Question: Was this GPS loss behaviour or low battery behaviour. If the latter, I really was not expecting it to do that.