http://www.faa.gov/pilots/lic_cert/p...OIA_and_PA.pdf
1-2. Actions NOT Reported By PRIA:
PRIA does NOT provide information concerning accidents or incidents in which the pilot may have been involved, any type of administrative action such as a warning letter, as well as enforcement cases that are still open, pending, cases under appeal, or reopened, when reporting on a pilot’s performance record.
I can tell you from personal experience that this is often not the case. In fact, I've had employers approach me to say they received my records, including my "warning letters," long after the they were to be expunged. I used to carry those letters to interviews to ensure there was no misunderstanding.
I've seen it happen with others, and I've experienced it with new hires while processing PRIA paperwork during the hiring process.
Regarding previous counsel by one poster that one might be unwise to disclose a letter of warning in an interview, do so at your own peril. If an employer asks if you have ever received a violation, letter of warning, etc, then you had better admit it up front. Having been asked and having denied the fact may be a firing offense later when it's discovered that yes, you did have such an administrative action.
Far better before ever getting to that point, when one may be the subject of an investigation or enforcement action, SEEK LEGAL COUNSEL.
Those here telling you not to do so are FAA employees. You really ought not find yourself about to be interviewed by the police, and believe them when they say "only the guilty need a lawyer. You're not guilty are you?" These are the wrong people to be seeking counsel regarding any regulatory matter, and especially regarding seeking legal counsel.
They're also the ones least affected when you become the subject of enforcement action. JNB (et al) suffers a minor inconvenience while doing the job he's paid to do. In other words, pursuing enforcement action is part of his job. As a career pilot, it may be the end of your job. It may affect you for many years to come, and even if it delays your career a few years, that's a few years off the top end at your most senior wages, which could cost you a great deal of money.