I have seen several adsense ads that auto-play. But it could be that they're violating Google's policies. I tried looking for a policy document, but this is all I could find.
I saw a few that expanded to take over the screen if you hover over them for like 5 seconds but they gave you a warning. I haven't seen autoplay though. Are you certain they were adsense? Sometimes ads can appear to be adsense because they are served via DoubleClick For Publishers ( https://www.doubleclickbygoogle.com/solutions/revenue-manage... ). The site controls the approval of those ads. Google won't approve an as that violates the policies unless it somehow slips by.
Its not something that happens very often, but I've seen it mostly on websites that are usually low quality/aggregation websites. Here's one such instance.
I have seen several adsense ads that auto-play. But it could be that they're violating Google's policies. I tried looking for a policy document, but this is all I could find.
https://support.google.com/adwordspolicy/answer/176108?hl=en