This may not apply to everyone but one thing that is good to check if it is on, Location Services, specifically if GPS is being used.
If the System Services uses GPS, the default is to not notify you that it is being used. But one side affect of this is, your iPhone will get warm/hot the longer it is used and you are not aware of this.
Check Settings > Privacy > Location Services (Scroll all the way to the bottom) > System Services.
At the bottom will be option for Status Bar Icon, default is Off. Turning this on, it will show a status icon when GPS is being used by System Services.
The one thing I notice is Traffic tends to randomly turn GPS on every so often for mine (Even if Apple Maps is not used). And it's used specifically for Apple Maps, traffic feature. If you never use Apple Maps Traffic (Which imho sucks), turn it off.
I tend to use Google Maps more and Google Maps traffic does not need that Location Services Traffic to be toggled on.