United States of America
Practice Relating to Rule 68. Precautions while Receiving Parlementaires
Section B. Detention of parlementaires
The US Field Manual (1956) states:
In case of abuse, [the commander to whom a parlementaire is sent] has the right to detain the parlementaire temporarily.
In addition to the right of detention for abuse of his position, a parlementaire may be detained in case he has seen anything or obtained knowledge which may be detrimental to the enemy, or if his departure would reveal information on the movement of troops. He should be detained only so long as circumstances imperatively demand, and information should be sent at once to his commander as to such detention, as well as of any other action taken against him or against his party.