Plural marking is usually contingent on definiteness (marked either by a definite article, or by a possessive pronoun). However, the discussions in Dijkhoff (1983), Vos-de Jesus (1989: 82f) and Kester & Schmitt (2007) show that a plural noun without a definite marker is acceptable in a context where the referents are familiar.
Vos-de Jesus explains the contrast between e baka nan and baka nan as follows: The former must refer anaphorically to a group of referents previously introduced in the discourse, whereas the latter must refer to a group of referents familiar to both speaker and hearer. Dijkhoff refers to the use of "existentially asserted NPs" as requiring a plural marker. Kester & Schmitt (2007) refer to such forms as "disguised bare plurals", which have a definite feature.
Source: Vos-de Jesus 1989: 82-83
Source: Dijkhoff 1983: 218
Source: Dijkhoff 1983: 220
Source: Kester and Schmitt 2007: 116
Source: Maduro 1971: 9