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

So much about the STR, MTR or LTR approach comes down to your individual goals, possible pain points and preferred investing prescriptions.

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.

You'll either need to acquire/reacquire prescriptive easement rights, negotiate a license or sue to quiet title if an abandoned claim. 

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.

Part of the agent's role is to supervise to be sure that nothing bad happens, or to work to rectify it if it does.Either way, always advise the tenants well in advance (according to the terms of your lease) and tell them to secure all valuables (money, jewelry), prescription and weapons.I also recommend minimizing the intrusion on the tenants by scheduling group showings at specified times only.