If a listing agent represents a seller and an unrepresented buyer wants to view their listing. That listing agent or an agent acting on the listing agent's behalf may show the property without a "Buyer's Agreement". Reference Site (Article 61): https://www.nar.realtor/the-facts/nar-settlement-faqs
Other situations include Open Houses, Rental Showings initiated by a property management company/landlord, and various tasks like property reports and opening the doors for inspectors and appraisers. Non-NAR affiliated real estate agents in states where Buyer Agreements are not required would also not be required to have a Buyer Agreement.