On the basis of the statute mentioned. I don't think "non-retainable" will be properly understood, but "may not be retained" should be. The relevant law states that the documents in question may not be retained by anyone, public or private, so we can't put "by the authorities", for example. I don't it's necessary to put anything after "retained"; it will be obvious that it refers to anyone other than the holder of the document.

The same phrase is used in the same sense in these explanatory notes to a UK bill:

"60. A travel document may not be retained beyond 14 days beginning with the day after the day the document was seized, unless that period is extended by a judicial authority (paragraph 5(2)). Documents must be returned to a person as soon as possible where none of the above reasons for retaining them continue to apply or where the 14 day period has expired, unless they can be lawfully retained under another power or provision not in Schedule 1 or detained for criminal or deportation proceedings under paragraph 7 ( p aragraph 5(3)). Paragraph 5(4) provides that the constable in possession of a person’s travel document must explain to him or her the grounds on which the document can be retained. [...]"
https://www.publications.parliament.uk/pa/bills/cbill/2014-2...