We may have found evidence of drug use on the job–what now?

Q: “I am an HR Manager at a big nursery with hundreds of greenhouses, and we provide our employees with portable toilets. It was brought to my attention by the cleaner that he found a cut-up soda can and a syringe in one of the potties. I have never encountered any drug use at our nursery, and I am not sure how I can handle this situation.” – J.