IPad charges itself?

This is really intriguing to me, but my iPad Air 3 has this tendency to charge overnight by itself even if it's not exactly plugged in. Let me explain it in detail. So I usually plug in my iPad at around 70~80% or sometimes a bit more for a few minutes(~30 minutes max) before going to bed. On unplugging it right before bed I turn off my wifi and bluetooth and switch it off/shut it down entirely. Usually upon waking up after an elongated time duration (~5 hours) I find that the charge level is higher than the original charge level I switched off my iPad at. For example, the charge level at the time of shutting it down last night was exactly 84%(this is 4:30A.M. in my time one). Upon waking up and switching it on it was 92%(9:30 A.M. in my time zone). The increase is usually only around 5~8%. This has been happening for as long as I can remember, and my iPad isn’t really too old as it was bought in early November last year (Nov 2019). Is this because of some battery issue, and should I be concerned about? Thank you!