There may be parts of the patent that are kept secret? but if they want full patent protection then it has to be published before the full prior art protection comes into play, if someone else stumbles upon the secret portion of the patent and they patent it themselves fully, then they would hold the patent and the prior art for the part that was kept secret.
Publication
Patent applications are generally published 18 months after the earliest priority date of the application. Prior to that publication the application is confidential to the patent office. After publication, depending upon local rules, certain parts of the application file may remain confidential, but it is common for all communications between an Applicant (or his agent) and the patent office to be publicly available.
The publication of a patent application marks the date at which it is publicly available and therefore at which it forms full prior art for other patent applications worldwide.
