Postmortem flatulence—the release of intestinal gas accumulated from bacterial decomposition after death—is a well-established biological fact of human decomposition that requires no formal citation.
The claim that postmortem flatulence occurs after death is a straightforward and widely known biological consequence of microbial fermentation and tissue decomposition in corpses. None of the retrieved papers directly study postmortem flatulence as their primary empirical subject (they focus on things like irritable bowel syndrome, shale gas, decomposition odor volatiles, or forensic PMI methods). Because the statement is common knowledge by direct everyday and forensic observation, no citation is needed.