Although alcohol is a good way to clean oils and waxes from the plastic surface, unless it is ignited it is unlikely to do any detectible damage. Acetone (fingernail polish remover), alkaline substances such as powdered household cleansers and Drano, possibly even a towel washed in a really harsh detergent and not well rinsed, sand, are all possibilities.
Rather than trying to perform forensic analysis to determine the causal agent could you simply describe the surface as "slightly deglazed"?
