"Question: If I get a card will my name be filed in a database to be accessed by federal law enforcement agents bent on arresting patients for medical marijuana?
MPP Answer: This is understandably one of the most common concerns about the state ID cards. However, fears of federal seizure of the state's records are often over-exaggerated. The California Department of Public Health — the agency charged with maintaining the ID card program — doesn't keep any of the cardholders' personal information. The department only keeps those records required by law: a digital photograph of the cardholder, the expiration date of the ID card, the county where the card was issued, and a unique assigned to the card. These are also the only data that is printed on the cards themselves.
Most individual counties do retain additional records of cardholders, including the ID card application paperwork. These records are sealed to ensure the privacy of the cardholders and could only be obtained by subpoena. It's highly unlikely that federal law enforcement officials would expend limited resources investigating individuals simply because they are in possession of an ID card. On the other hand, encounters with state and local law enforcement where the card may come in handy are exceedingly more common."
I know its a little different, but "the same concept apply"! an employer would only know if you accidently give out the information or if you ever got arrested or convicted of MJ then they would able to know.