Several times during the meal, the customer takes action that ensures that others know of his presence (he asks someone to pass the salt, etc.).
The party agree to meet with the customer the next day in the same tavern, at which point the customer promises to relate some information that would interest them.
The next day, however, the customer never shows. When the party inquires about him, they are told by everyone that the customer didn't exist; that the party never interacted with anyone but themselves.
One of the party members finds something that conspicuously belonged to the customer.