Why does iPhone 11 battery drain by 2% when switched off?

Hi guys!

I noticed a recent trend with my iPhone 11 battery two days ago. It started like this: I charged the phone up to 100% and switched it off immediately to go to bed. I woke up the next morning, switched it on, and it was at 98%. I checked the battery health - still 100% capacity. I checked the time of last charge and truly, it showed it was charged up to 100% the previous night. A few minutes later, it changed and showed it was charged up to 98% as at the time I woke up.

Last night, I charged it up to 64% and switched it off. Woke up and turned it on, it was at 62%.

As a bit of background, I bought the phone fairly used. The battery is still at 100% capacity though I read somewhere about battery recalibration after a phone has been idle for a while. So I think I’m expecting it to drop to its true battery health after a while. I’ve only had the phone for a week.

I’m currently on IOS 16.0.2. I’m not sure I have any settings that would demand the phone to keep running after I switch it off, especially since I swipe away all apps from the task manager and close all notifications before switching it off. My WiFi, Bluetooth, and hotspot are also always off till I turn them on to use.

This is an issue I never noticed up until two days ago. But asides from it, the battery and performance of the phone is great. I just need advice on what the cause of this recent development could be and how I can fix it or whether I need to go change the phone (I have a warranty for one month since it’s a used phone, so I have three weeks left on it).

Looking forward to hearing from you guys!