iPhone battery still going down even though I’m not using it

If it is powered on then there are a lot of processes running that do not show in the Settings, Battery chart. Spotlight, email and social media polling, Photo analysis (so it can give you a lovely memory or find all the bikes in advance) etc.

If you suspect a battery fault then do this test.

Charge to 100% before retiring for the night. Take off charge. Power off using the button and slider.

Power back on in the morning and after a couple of hours check the Settings, Battery chart. Usage should be only a few% during the off period. (boot up uses battery)

That's what batteries do when being used.