Here are some assumptions
1) It is out of its mind. Don't laugh. I mean the flight control program is performing some procedure not as designed. I do believe that the "fly away" is not in the specification of flight controller. When lost signal it should return home or landing as setting. Lost GPS reception it goes ATTI and wait for stick command not flying away. It may drift a bli but still there. Compass error does not initiate a fly away if it is not going somewhere, RTH for example. Anyway by the system design the RTH or any on-going mission should be aborted when compass error. So the only explaination is it is out of its control path.
2) RC or Mobile device send command(s) causing Spark to fly away. I saw this once on Youtube. A man directly connect mobile device directly to Spark, command Spark to Active track then put the mobile into his pocket then Spark starts flying away. Putting mobile into pocket might caused virtual joy stick to work unpredictable. Incompatible mobile runnung Dji Go 4 may send serirs of unpredictable commands.
3) RC malfunction
4) Bug in specific function and engaged under certain condition causing a flyaway. Someone report a flyaway when perform rocket.

Yes, if wifi lost signal should be automatically rth mode, but my spark flies away, and lost it don't know where, I'm very sad and no more confident to play drone now

Newbie here, has anyone had a Fly Away when flying in Beginners mode?

Thanks for the sharing. Sometimes it depends on a certain flight and we need to evaluate from the flight data. So if someone had this case, we would recommend contacting our support and we will help to start a data analysis.

Andy6178 Posted at 2017-8-5 09:34
Yes, if wifi lost signal should be automatically rth mode, but my spark flies away, and lost it don't know where, I'm very sad and no more confident to play drone now

We are sorry to hear about your case. Please contact our support to start a ticket, we will help to make data analysis, if it's malfunction, we will compensate your drone if it is under warranty according to After-Sales Policy.
Here’s the link to contact us: http://www.dji.com/support