Unfortunately they were a couple of drug addicts and they had to leave after only one month.  

If I'm getting calls from only 1 tenants about drug activity, or people living in a unit not on a lease, what is the best way to proceed?

Living next door to a drug infested apartment complex.

Parks tend to have drugs, criminals,transients.

I have seen a fire, serious domestic abuse, drugs, tenants from hell, and it hasn't gotten me running yet.

Check the neighborhood at night to see where the drug dealers are dealing and stay away.

He has continued to pay rent, but he has:Played loud music after midnight (Lease rule addendum violation)An unauthorized occupant (Lease Violation)Smoking in the apartment (Lease Violation)Possibly the unauthorized occupant is selling drugs.

I know where all the gangs are the high drug areas and I know a lot of people native to many areas in Detroit...

A bigger problem too is that they may be dealing drugs.

My manager has 15 fourplexes that he manages in the subdivision out of about 60 blogs.I'm concerned about the fact that it is a low income area with the usual problems - drugs, petty theft, domestic violence, etc but I don't feel uncomfortable when I go there and my manager says he is not uncomfortable day or night.