Same happened on the last night, during 5hours 60% disappeared. I charged my 6S+ on the last evening, browsed abit... Left it around 24-01.00 on the table (~90%) and today on 6.00 the battery was 28%. No apps showing the usage in the battery stat. And of course it happened now when im not going straight to the work and need my phone as "full charged".. Still 30mins left to charged it as much as it can...
At first i thought that it was youtube app.. I do remember reading something about that app and the battery.. But the battery stat doesnt show any hint of that... Blaaah!
Edit. The difference is that i used the ipad charger on the last evening to charged the phone from 80% to full...