OK. I experimented last night by turning AOD(Always-On Display) and Wifi off. and it stopped the battery drain.

I check with battery and CPU usage monitoring app. I don't know which one was causing battery drainage, probably Wi-Fi. I think Samsung did not test their S7 enough with this Qualcomm Snapdragon AP (this one chip has everything  in one such as CPU, GPU, 3G,4G,Wifi,Bluetooth,Battery Management, etc.,). Only USA and China versions have Snapdragon AP , all other countries' version uses Samsung Exynos AP.

Hopefully they will fix this problem by releasing firmware update.