I did some further research into this and I found out that upon completion of the diversion program my case actually was not filed. The county in CA where I did pretrial diversion, Santa Clara County adopted a new policy on petty theft diversion for first time offenders in July 2012. If the diversion program was completed successfully then the case would not be filed in court.
I confirmed this by looking at the Santa Clara County case index of criminal cases filed in court, and my name/case number had no history available.
So if my case is not even filed in court, can I say no to a deferred prosecution?
Divorce / Separation Lawyer
In Colorado diversion and deferred prosecution are also different. I believe based on the information you provided that you can honestly answer that you have not had a deferred prosecution. If for some reason they find this other case, they may fire you for the what they feel is not an honest answer, however, you can certainly point out that you weren't sure how to answer, so you researched it and that looked correct. *(However, you will probably be an at will employee which means you can be fired at any time even without a reason, so their ability to fire you if they don't like your answer, really doesn't matter.)
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