Postpositive "unknown"

There are some single adjectives that can occur postpositively. Your example is like those in [12i], below, in that the postpositive construction alternates with the attributive one with the same meaning. It is not possible to predict, on some general grounds, which adjectives can be used this way; some can, others can't, and the best we can do is record, for each adjective, to which category it belongs.

In addition, as was mentioned in the comments, a person or persons unknown is a cliche that is close to becoming a set phrase in some contexts.

Here is the relevant discussion from CGEL Ch. 5, §14.2 (pp. 445-446):

(b) Adjectives and AdjPs

The basic rule for the placement of adjectives and AdjPs is that single adjectives and phrases without their own post-head dependents occur in attributive position, while others occur postpositively, i.e. after the head of the NP, as in members [dissatisfied with the board's decision]. Special rules apply with compound determinatives like somebody, anything, etc.: see §9.6. For the rest, there are certain specific adjectives that can or must occur postpositively. Representative examples are given in [12] (see also Ch. 6, §4.2):

[12]  i  the only day suitable, years past, proof positive, matters financial,
             all things Irish
        ii  the people present, the cars involved, the students concerned, the city proper
       iii  the heir apparent, the body politic, the president elect, the devil incarnate,
             the poet laureate, a notary public
       iv  the house [currently ablaze], all people [now alive], the ones asleep

The examples in [i2i] alternate with the attributive construction: compare the only suitable day, etc. The postpositive use of these adjectives is subject to severe restrictions. Adjectives in -able or -ible, like suitable and possible, require an attributive superlative or only, compare the best result possible and *the result possible. Postpositive past occurs with temporal nouns (cf. *approaches past), and positive only with proof. The last two examples have general nouns as head and denote domains; the adjectives financial and Irish are like restrictors (§9.6) in that they must immediately follow the head.

The adjectives in [12ii] occur both attributively and postpositively, but with a difference in sense. Postpositive present (or absent) denotes a temporary state of affairs: compare the present government. The same applies to involved and concerned, though here the attributive sense differs more (cf. deeply involved activists, concerned parents). Postpositive proper means "in the strict/proper sense of the term". With net and gross the choice of position depends not on the meaning but on the head: they follow specific sums (fifty dollars net) but otherwise precede (my net income).

The examples in [12iii] are fixed phrases, with no attributive alternant. The adjectives in [iv] are ones which are altogether excluded from attributive position.