I used to build roms and saw this behavior on occasion by people using those roms.
First thing I always look at is the widgets. You may have setup the phones with the same apps, but did you setup the widgets the same way? Specifically news, weather and social media widgets. Worse, do any of them geolocate and/or have ads? These will absolutely murder your battery, and no, it will not show up in battery stats. Why they don't show up I have no idea, but I've used multiple battery monitors and such, it rarely registers, but as soon as you start killing those apps or denying them access while idle the battery stops draining. Just because you installed them doesn't mean they triggered, they need to be setup and launched at least once. Another thing to check is to reset the battery stats, if they get messed up, it can make the battery think it's empty even when it has a lot more battery remaining, this is more common for people swapping roms, but it can happen to a stock phone as well. Lastly, as mentioned, the cell signal (not wifi). When your phone is an area with no signal, it will turn on all radios and bands trying to locate a signal, any signal. This will also drain your battery like a vampire and is very often why the phone gets hot while idle.
Beware what you install, particularly widgets. On weather apps, choose ones that let you set a static location rather than pinging the gps. Get rid of social media, particularly the widgets, do you really need to know the instant your aunt uploaded a new recipe? Buy them, firewall them or use an adblock, or better yet, use all three. Watch who is making your apps. Run a light weight launcher, the more slick features it has, the worse it's likely going to be, that goes double for the stock launcher on a Samsung. Disable radios you are not using or can't use, many GSM phones have a cdma option and it will try that radio if you lose signal even in a country that doesn't use CDMA.
A word about battery monitors... You're trying to find what is running and draining the battery by installing yet another another app that constantly runs and checks what is running and monitors battery. Some will even block apps which needs even more power and since you probably only want it for a short term, it's probably free, which means it's going to downloads ads in background to cache them and pull gps info. You may be your own worst enemy here as these also (surprise) can drain your battery, particularly ones that spy on you. Most of the time I can figure out what is draining better than these apps by simply doing what I posted above. If you do want to do this, do it them get rid of the app, I don't believe in using a battery monitor long term, it's just silly. If an app is causing problems, get rid of it, don't install another app to control it.
Biggest tip of all...
Root (and/or rom) your phone, all those default installed apps are terrible, even default ones you like. Most probably updated as soon as you turned on the phone, but since it's preinstalled that means it was not replaced the phone simply put a newer copy in the data partition. Is the old one hurting anything? Besides soaking up space, causing longer boots (due to how Android caches) and posing a threat risk, it's dragging down your phone. You can strip out a stock rom and people will be amazed at how fast and efficient it runs and all you did was debloat it and reinstalled the same apps they used before. You can tell me till you are blue in the face that these updated default apps do nothing, and you can disable them all you want, it's still not the same as removing them or never having them installed in the first place. People like to think roms make the phone faster because of tweaks by the developer but the truth is almost all of that speed is simply because it lacks bloat, I've done it, I've tested it. Think about a preinstalled Windows verses a fresh install, anyone who has done it has seen the difference, even compared to how it runs after uninstalling things, it's just not the same. "Root is a security threat", maybe, but my rooted older phone has a newer Android version than your newer phone and I have tools at my disposal to deal with it so I'd argue its a wash at worst and if I was worried I could always unroot my rom (yes, it's a thing). If a non rooted phone gets owned, you're buying a new phone, or rooting it to fix it anyway.
Edit: Forgot one other thing... Google services. Every now and then they will issue an update and it clashes with existing data, the fix is dump the data or uninstall the reinstall that.