Related Rule
Practice Relating to Rule 106. Conditions for Prisoner-of-War Status
Section C. Situations where combatants cannot distinguish themselves
Switzerland’s Basic Military Manual (1987) states:
Exceptionally, for example, in the case of guerrilla warfare, combatants are not obliged to wear a uniform or a distinctive sign. They are considered as members of the armed forces who have the right to prisoner-of-war status, provided they fight for a State or a liberation movement, within an organization which has a responsible command and a disciplinary system, and provided they carry their arms openly before and during an attack. 
Switzerland, Lois et coutumes de la guerre (Extrait et commentaire), Règlement 51.7/II f, Armée Suisse, 1987, Article 26(2); see also Article 64.
At the CDDH, Switzerland abstained in the vote on Article 42 of the draft Additional Protocol I (now Article 44) because it was afraid that “the article would only have the effect of doing away with the distinctions between combatants and civilians. The consequence would be that the adverse party could take draconian measures against civilians suspected of being combatants.” 
Switzerland, Statement at the CDDH, Official Records, Vol. VI, CDDH/SR.40, 26 May 1977, p. 131, § 68.