There are countless possibilities. Responsible food production operations take steps to keep cleaning supplies etc away from active production lines prior to final packaging. After that it’s out of their hands.
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Formaldehyde added to foreign beer (purported)
For years there have been reports or rumors that some/all brands of beer made outside the US contain a large amount of added formaldehyde. I ran GC analyses on some from around the Pacific Rim and found no detectable levels. I communicated with the AOAC Referee for standard analysis procedures for alcohol beverages (S. Dugar) and he passed along useful information on the state of the art for analytical methods. Later while further researching the topic I did surprisingly find references about this (adding formaldehyde at the various stages); however, this is not a common practice anymore. At the end of the linked page it shows a survey on 11 types of alcohol/liquor. The only products that had zero positives were beer and vodka. Apart from deliberately added formaldehyde, fruits contribute methanol and formaldehyde (much more so than grains) due to the pectin content.
https://danharring54.blogspot.com/2022/09/formaldehyde-added-to-foreign-beer.html
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Musty odors picked up during transportation and storage. 3 types.
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Cleaning chemicals
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Solvents with volatile fumes
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Petroleum products. Light compounds that evaporate easily, middle range liquids, viscous oily lubricants, waxes
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Packaging additives/contaminants
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pesticides
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Personal care products (can be tasted at extremely low levels when warmed in the mouth). Hand lotion, cologne, vaseline, suntan lotion, soap…
Dan Harring (54) 