if you are applying for a job at a large company, or any company that has any contracts with the government, then it will show up on a background check. Its amazing that some people think that the government will respect privacy laws. Also most PI's (companies hire them to do the background check) have connections to law enforcement/ government officials, so its really pretty easy to find it out.
The companies will find out and though they can't cite medical marijuana as the reason you are fired or not hired, it will be the reason.
Also unless you pay cash every single time you buy weed, or paid cash when you got your medical liscense, it show up on any basic background check.
I have friends who have cards, and they work for their parents,or live off the grid, or can't find a non-shitty job.