Related Rule
Practice Relating to Rule 106. Conditions for Prisoner-of-War Status
Section C. Situations where combatants cannot distinguish themselves
Mexico’s Army and Air Force Manual (2009), in a section on prisoner-of-war status under the 1949 Geneva Convention III, states:
146. …
In exceptional cases, when required by the nature of the hostilities, combatants may be released from the obligation to distinguish themselves from the civilian population by wearing a uniform or a fixed distinctive sign recognizable at a distance during military operations. However, in such situations, these combatants must distinguish themselves by carrying arms openly during the engagement and during such time as they are visible to the adversary while engaged in a military deployment preceding the launching of an attack in which they are to participate. Failure to comply with the obligation to carry arms openly can deprive combatants of their status, but not of the rights and guarantees relating to it, if they are prosecuted for carrying weapons illegally either with or without other offences.
148. To avoid uncertainty and prevent any arbitrary measures at the time of capture, it is provided that persons taking part in hostilities and captured must be presumed to be prisoners of war and treated as such, even in the event of doubt as to their status. 
Mexico, Manual de Derecho Internacional Humanitario para el Ejército y la Fuerza Área Mexicanos, Ministry of National Defence, June 2009, §§ 146 and 148.