My go-to for situations like these: Assume that the offender _clearly_ didn't mean to behave incorrectly, and help them overcome the mistake.

Person in a public space listening to reels at full volume? Get their attention, then loudly point out that their headphones got disconnected and everybody can hear the audio.

People leaving a train or bus and leaving behind trash? Loudly let them know that they forgot their water bottle or paper bag. If it's a single item, this works doubly well if you helpfully hand them the item, too.