Giving seat away (some within CoC, some not):
Alaska: https://thepointsguy.com/2016/03/whe...ive-seat-away/
American: https://thepointsguy.com/2018/02/sea...mistake-story/
Delta: https://money.cnn.com/2017/07/17/new...eat/index.html
Delta Gave Away My Assigned Seats
https://www.airfarewatchdog.com/blog...way-our-seats/

Early departure:

Delta: Delayed flight left early. Proper?
https://crankyflier.com/2016/11/14/w...te-ask-cranky/

American: Missing delayed flight that left early, no announcement: issues, recourse

JetBlue: Plane left early without ticketed passenger (being me!)

Frontier: https://www.elliott.org/is-this-enou...alk-away-gate/

Southwest: delayed and then pushed back early so i missed my flight

Continental / old United: https://www.farecompare.com/news/gat...flight-delays/

Keep in mind that most international airlines operate under a positive passenger-bag match requirement. If they're going around looking for you, it's not because of their increased commitment to customer service, it's to avoid having to pull your bag from the flight and delay everybody. Domestic US flights have no such requirement, so US airlines are less likely to lose sleep over a missing passenger.