I answered this question which had a bounty (100 or 150 I think) but now it doesn't?
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I'm guessing that the bounty period probably expired without the owner of the question accepting an answer.
There are occasions where the auto-accept doesn't kick in as I'd expect; for example, there was a bounty that expired on this question a couple of hours ago, but no bounty was awarded, and no answer has been auto-accepted.