How can I pinpoint the cause of sudden battery drain on a relatively new phone

[Brian81]

My smartphone's battery has started draining dramatically faster over the past few weeks, often losing over 30% overnight with minimal use. I've checked the battery health report, and it shows normal capacity, so I suspect a software or background app issue. I've already tried the basic steps like restarting and checking for obvious battery-hungry apps. For others who have solved similar mysterious drain issues, what deeper diagnostic steps or specific settings did you find that pinpointed the culprit, especially on a relatively new phone that shouldn't have hardware problems yet?