@Jan *****************
it really depends on the agent and how experienced they are. Those working with the airlines for a while know that return ticket rule is not enforced by Thai immigration, while a new agent can just read off the list of requirements and is not familiar with different types of visas. It can also happen that one of their passenger was not permitted in Thailand by authorities and airline was fined and had to fly them back so now they are more cautious. As a rule, airlines are the ones checking all required documents including passport validity, visas, return tickets etc - before checkin