Just because a petty misdemeanor does not show up in a criminal record does not mean it may not haunt you if you switch jobs. The reason is that employers conduct background checks, and my understanding is that they can go really deep into a background check.
Thus, a background check could reveal your misdemeanor if the employer wanted to be really thorough. A superficial background check will not show it however. All misdemeanors are not the same in an employer's eyes too. He will view a petty theft with far more seriousness than a traffic ticket.