In other words, the use of words ending in -ive as nouns has a very long history.[...] the suffix is largely used in the modern Romanic langs., and in English, to adapt Latin words in -īvus, or form words on Latin analogies, with the sense ‘having a tendency to, having the nature, character, or quality of, given to (some action)’.
Already in Latin many of these adj[ective]s were used subst[antively]; this precedent is freely followed in the mod[ern] lang[uages] and in English: e.g. adjective, captive, derivative, expletive, explosive, fugitive, indicative, incentive, invective, locomotive, missive, native, nominative, prerogative, sedative, subjunctive.
According to the Oxford English Dictionary detective comes from the English detect plus the suffix -ive. The OED and the Trésor de la Langue Française Informatisé both say that the French word for the investigator, détective, is derived from the English noun.The word 'adjective' is a noun itself. I don't know what the rules are, but I suspect all -ive words are French loanwords (e.g. primitive, objective, productive, etc.), so you may better ask a Frenchman.