A reasonable approach might be to not volunteer any information about your polygraph experience, but if asked, to simply mention that you were polygraphed once before but were never told the outcome. I think it would be prudent to mention nothing more, unless specifically asked. And it would be entirely counterproductive for you to imply to your polygrapher that you think you must have failed the prior polygraph "test."

Several years ago I took a polygraph for a law enforcement position.  I was never officially told that I failed, but since I got the old post-test interregation and a form letter telling me my application was withdrawn (no reason given).  I wasn't terribly upset becasue I already had a great job that paid twice as much as the law enforcement job.  However, I did (and still do) want to serve my community, so I have applied to another agency.  With a new polygraph on the horizon I know that I will be asked about the old one.  Any advic on how I should answer questions regarding previous polygraphs?