There is a whole new crop of synthetic marijuana type products on the market that are pretty popular with the younger generation. Many are legal, but states are doing what they can to try to get on top of it and add them to controlled substance lists. Many of these products could have an odd smell to them that doesn't necessarily smell like marijuana or tobacco. There is also another group of substances that are in the "bath salt" group. They label them "not for human consumption" but they are being used for just that.

Educate yourself by doing a search for either bath salts or synthetic marijuana. This might point you in a better direction.

Also, the guy that came to see you needs to be alert to behavior and physical changes in the suspected users. That might clue him in more.