I would guess at indole. I have met it in commercial perries in the past. See here from a book chapter of mine some years ago...
A nontraditional taint in ciders is that caused by indole. This compound is well known in meat products, particularly pork, where it can form a part of the so-called boar taint and is derived from tryptophan breakdown (Wilkins 1990). At very low levels it is also found in many flower aromas, and indeed it is often incorporated in soaps and perfumes for its floral attributes. At levels in excess of 200 parts per billion, however, its odor becomes increasingly faecal and unpleasant. Unpublished work in the author’s laboratory identified indole as a relatively widespread taint in ciders, which may derive from an odorless precursor or salt since it often appears and disappears from bottled products. Almost certainly it is not derived from tryptophan, since this amino acid is virtually lacking in apple juice, and in ciders no trace has been found of skatole (3-methyl indole) that would be a necessary intermediate. Current belief is that it is generated de novo by the yeast from inorganic nitrogen during its own synthesis of tryptophan, rather than its breakdown. The factors favoring the synthesis of indole appear to be a low juice content and a low yeast pitching rate, coupled with a fast fermentation stimulated by high temperature and the addition of simple inorganic nutrients such as ammonium phosphate. Under these conditions, the yeast vitamin requirements are not adequately met from endogenous sources, and a specific deficiency in pyridoxine (a known co-factor in transamination reactions) appears to be the immediate cause of indole formation. Industry sources suggest that indole formation can therefore be suppressed by the addition of pyridoxine to the juice at ca. 1 ppm.
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Gabe Cook likes to jokingly mention the sausage flavored perry he once ended up with. Well I am currently encountering that flavor too. It's not what I want in my perry. Wondering if it ages out. And what circumstances contribute to it.