I think you are spot on. The Cpl has Weapons Platoon or Company sticker. He is probably going on terminal leave the next day, is a counter-intel Marine, or they are on MSG. These are all Marines in the Dayroom. Short timers like to rebel and not wear a skivvy shirt under cammies.
Perchloric Acid K.P. Baran, S. Gad, in Encyclopedia of Toxicology (Third Edition), 2014 Background Perchloric acid is a corrosive inorganic liquid that is colorless, odorless, and oily in nature. It is miscible with water and has a boiling point of 203 °C, a melting point of −19 °C, and a low vapor pressure of 6.8 mm Hg at 25 °C. A cold 70% aqueous perchloric acid solution is considered to be a strong acid or superacid (stronger than sulfuric and nitric acids) but is not necessarily a strong oxidizing agent. However, as the concentration and temperature of the perchloric acid are increased, so does its oxidizing power. Hot concentrated solutions of perchloric acid can be extremely dangerous (explosion hazard, fire hazard). Perchloric acid can react violently at room temperature with cellulose materials (e.g., wood, paper), oxidizable organic materials (e.g., alcohols, ketones, and aldehydes), and strong basic materials, resulting in an increased risk of fire and/or explosion. When heated, perchloric acid can decompose and generate toxic and corrosive fumes. Many types of metals can react with perchloric acid to produce flammable/explosive gases. Because perchloric acid becomes increasingly unstable (volatile) with increasing concentration, it is not marketed at concentrations greater than 72% v/v in water. It may explode by shock or concussion as an anhydrous material (e.g., if a spill of 70% perchloric acid is allowed to dehydrate). Mixtures of perchlorates with oxidizable substances are often explosive. Perchloric acid is attracted to water and disassociates readily to yield the perchlorate anion. The toxicity of perchloric acid comes from the toxicity of the perchlorate anion. The potential danger of working with perchloric acid is sadly illustrated in a workplace tragedy. A mixture of perchloric acid and acetic anhydride exploded in a Los Angeles factory in 1947, killing 15, injuring 400, and causing $2 million in damages.
Same on MSLSD—panic!