I think I somehow bugged my phone.
The first part was normal and standby. Then when it starts falling I played Symphony game for about 30 minutes.
Pressed close all in recents.
Used the phone as media playee for 2-4hours.
And then... Nothing! The phone just continued to drain battery like that. Even tough I did nothing! BBS did not show wakelocks. But I had roughly 3% per hour.
I mean just being in a 4G network can't just drain the battery if the phone is idle...
I assume symphony app did NOT correctly close and kept running in the background.