I think I'll have to take my 7+ Back as the standby drain is terrible. I'm losing between 8 and 10% battery in Airplane mode with WiFi enabled over night. The SIM card isn't even in my 7+ as I don't have a screen protector yet so I'm still using my S7E but even the edge doesn't drain 10% over night and it's notorious for excessive standby drain.
Even checking the battery usage doesn't show any suspicious behaviour except for the Settings app which was on for 34 minutes in background.
That doesn't seem right at all. So as an example on my 7+ since I adjusted the background app refresh of a few apps on a hunch - last night: I had the phone plugged in late evening. Took off. Used to was at 98% when I went to sleep. No airplane mode. I have do not disturb scheduled from 1AM to 5.15AM. When I woke, phone was at 96%. Have you installed apps? Are you updated to 10.0.1?