I swore I pressed the button to turn it off! Or maybe it got accidentally turned on? My pant pocket gets really warm and there goes its power left too. :(
If I understand your scenario correctly, I have had a similar issue on a couple older iPhones including the original SE.
Use the camera (either left swipe from lock screen, or via Control Center button) and then press Power to lock/turn off the screen, confirming the screen is dark before putting it away. Short time later or later in the day I will occasionally find the Camera app has drained the battery as if I were using it the entire time it was in standby, even it Low Power mode is active.
I have not found a pattern that would allow me to avoid this, but it works best when I go to the home screen (which may require an unlock) and/or check if Camera is running in apps (and close it if it is).
I’ve run into similar situations where the iPhone gets hot in my pocket and I realize the screen has been on.
If there’s a chance that your pocket is slightly damp, I believe that can turn on the iPhone’s screen and potentially tap buttons on it.
Due to the fact that I carry several devices with screens and I’m afraid of other stuff in my pocket scratching and cracking those screens, I’ve been carrying my phones (and iPod) in a leather belt pouch.
I am currently using a Potomac Leather Dagger Pouch (leaving the knife at home). The medium size has enough room to hold three large phones and I don’t have to worry about the screens touching anything other than the leather or other phones. And it’s really convenient to quickly remove and replace the phone(s) when I need to use them.
Is the root cause accidental side button press in your pocket or a damp pocket rubbing up against the screen? If it’s the latter, a workaround could be setting this
Settings > Accessibility > Touch > Tap to Wake > off
I have found mine awake and hot in a bike bag. I have it in a baggie there for some protection, and suspect it’s a combination of being jostled and then something hitting it and moving the screen to an app or something like the camera. I have the same issue when I put the Garmin in a bike back and that doesn’t go in a baggie.
I have Raise to Wake AND Tap to Wake turned on, so I guess no surprise there!
They do get REALLY hot in pockets.
Diane