I just took a polygraph exam for employment with the secret service and failed. If I apply to another federal agency that does not require a pre-employment poly exam, should I tell them of my polygraph experience and would they find out about my exam even if I did not mention it?

Just an observation. If the agency you are applying for doesn't use polygraph, there is a reason. Either they have no or little faith in it, or don't have an examiner available.  I suppose there are other possible reasons out there, but I think those would be the top two.  If asked you should be confident and state the fact that the polygraph accuracy really depends on the competency of the examiner, and then is no better than chance. Do some research into the agency you are looking at. Find out why they don't use the polygraph. You will be much better prepared to eliminate any fears a prospective employer may have about you, if you know their beliefs about the polygraph issue. If I'm not mistaken, police departments nationwide are having trouble filling openings. At a recent recruiting seminar, I heard the instructor advising that it was the potential employee's market.  If you're willing to relocate, you will find a good department that appreciates your history, verified by a competent background investigator, not a bunch of wires and rubber tubes. Hope this helps.