I'm reading the guidelines of the Oxford Dictionary of Phrasal Verbs. Here is the paragragh I'm reading:
A highly irregular group is made up of such items as unaccounted for, uncalled for, and unguessed at. Certain of these combinations have a corresponding finite form without the negative prefix, thus account for, call for, guess at, and this can be used in both active and passive sentences. Compare:
These missiles are unaccounted for by the authorities.
i.) These missiles are accounted for by the authorities.
ii.) The authorities have accounted for these missiles.
However, there is no such sentence as
The authorities have unacconted for these missiles.
to correspond, as active, to the first of the set of three.