You're anonymous in a metro train. You're not in a self-driving car. As others have mentioned, the app can have a button to report a cleanliness issue. Working

You're anonymous in a metro train. You're not in a self-driving car. As others have mentioned, the app can have a button to report a cleanliness issue. Working back from the first report means you don't have to review 12 hours of footage. As a side note (since this is HN), you don't have to review 12 hours linearly, you can use a binary search, which means you can probably find the culprit in minutes at most.