The agency with which you are seeking employment is unlikely to learn of your practice polygraph unless 1) you tell them or 2) the polygraph operator you hired reports you.

If you tell your federal polygraph operator that you took a practice test with a private operator, the likely result is that you'll be asked all about the practice test you took, including your motivations for doing that, and that you'll ultimately be accused of attempted polygraph countermeasures.

I am in the application process for a federal leo position that requires a polygraph test.  I have not yet been scheduled . Hearing all the horror stories from my friends about failing it made me anxious as well as reading and researching.  So i hired a private polygraph examiner and had him administer a similar format of that for federal leo IE direct lie polygraph.  I passed with flying colors
Here are my concerns.  I only did this to ease my worries and anxiety but:
They WILL ask if I've taken a polygraph.  I know better not to admit but what's stopping them from finding out.  Will they ? 
The examiner himself told me that I should tell them the truth and explain my reasoning.  Your thoughts