Can a police officer tell your workplace to drug test you if they have suspicion of drug use?

I was pulled over for expired tags and having a previous marijuana arrest, the cop asked to search my car. I agreed. The cop found nothing but drilled me on the last time i smoked marijuana. I told him it had been months but actually it was days... I had my name badge on from work and he asked if they had drug tested me. I told him they had. Before letting me go (with no tickets) he asked my birthdate again and wrote it on his hand. I'm worried he will call my work and tell them to "random" drug test me. Is this legal?