"(3) In the case of anonymous or pseudonymous works, the term of protection granted by this Convention shall expire fifty years after the work has been lawfully made available to the public. However, when the pseudonym adopted by the author leaves no doubt as to his identity, the term of protection shall be that provided in paragraph (1). If the author of an anonymous or pseudonymous work discloses his identity during the above-mentioned period, the term of protection applicable shall be that provided in paragraph (1). The countries of the Union shall not be required to protect anonymous or pseudonymous works in respect of which it is reasonable to presume that their author has been dead for fifty years."
Virtually everyone is a Berne Convention signatory; but actual implementation in domestic law has been both spottier and more...complex... than the convention text itself. It seems unlikely that something of clearly recent authorship would find itself presumed to be uncopyrighted merely because an author could not be found; but I'd imagine that, in practice, the more risk-averse would be very, very, jumpy about taking 'anonymous coward' at his word that they are authorized to use a given piece of code under the terms of whatever license, that he is even the author, and so forth. That might hinder adoption.