Possible Clues:

• Disguised gin and tonic cocktail round
• With identity kept secret, perhaps travelling thus
• Unknown stranger, noticing nothing
• On the sly, poured out gin and tonic, nothing more
• Unknown coin got in change
• Camouflaged and in disguise, noticing nothing
• Like compilers noticing anagrams with cry of surprise
• Noticing undone zip, unbeknownst to others
• Suspect noticing nothing in disguise
• With identity hidden