Let's clarify what is being talked about. The first construction that Ven is talking about --"noun + PPP"--is a noun plus a modifying present participial phrase, e.g.
1) The witness being interrogated right now doesn't seem to be very cooperative
2) They've just arrested a man answering the description of the killer
This construction--a noun plus a modifying present participial phrase--can function as the grammatical subject, the direct object, the indirect object, or the object of a preposition. That's because it is a noun phrase.
This present participial phrase can be considered a "reduced" restrictive relative clause: 1) "who is being interrogated right now"; 2) "who answers the description of the killer." It doesn't matter, by the way, whether the present participle is active or passive in form.
Restrictive relative clauses--and "reduced" restrictive relative clauses--modify noun phrases. The noun that is modified by the relative clause is the grammatical subject of the verb in the main clause., as can be seen in Sentences 1 and 2. Neither 1 nor 2 could ever function as an absolute.
An absolute plays a different role in an utterance, and therefore has a different structure. An absolute can consist of a noun followed by a present participial phrase, as in
3) Our package having been lost by the shipping company, we demanded compensation
4) The bankrupt owners walked grimly out of the factory, the idle machinery bidding them a sad farewell
An absolute does not modify anything. An absolute has a noun, but that noun is never the grammatical subject of the main clause, as can be seen in Sentences 3 and 4. The absolutes in Sentences 3 and 4 could never be used as postmodifiers.
Absolutes are substitutes for independent clauses, although they are not themselves clauses--independent or otherwise.