Someone is using drugs in our office; what do we do?

I worked late and was in the office alone.  I went to get a cup of coffee and found a hypodermic needle in the trash.  I didn’t know what to do. Was someone diabetic or was someone injecting drugs in the office?

I asked my partner what she thought. She was uncertain what to do. We decided we would wait and see what happened next. There was no signs of anything for a month of more. Then, we found another needle and some white powder. We called the police who tested the powder and said it was heroin.

How do we find out who was using drugs in the office? What do we do when find out?