For some reason, some tenants have no issue leaving paraphernalia out while a plumber visits but would hide that from an owner.Also be on good terms with your neighbors. 

Now you would think that a city would be grateful to a citizen who was not only improving one of it’s historical assets that was condemned and dilapidated, as well as a drug den with paraphernalia laying about, but also bringing an error to their attention.

Know what to look for in terms of evidence.... drug paraphernalia, odors, residue on walls and in furnace filters, etc.What did they say when you served them the prior notices to comply?

Squatters (if they are kind enough) leave buckets of human waste, drug paraphernalia, and even light bon fires in the middle of the living room, therefore, burning down the house in most cases. 

I'd give notice to the apartment, SO the grandma and mom know also, that you found noticeable use  of pot the (ODOR), paraphernalia left ( the pipe), and the woman just exiting the laundry room, when you went in.That this type of behavior is not permitted on the premises and it could jeopardize the tenancy of all.Get a Crime Free/ Drug Free addendum and have all on lease sign it you can add that at any time if you don't have one already.If you leave this unaddressed likely it will just be tested again.

I've also found drug paraphernalia, kegs, and things on stuff on the walls from someone's nose.

left the place trashy but not trashed.I also once "self-evicted" a tenant before I knew the legality of what I was doing but since I found the drug paraphernalia while moving his stuff, I don't think he was going to complain.

Initially after first talking to the cops, the drug paraphernalia was moved from my yard/basement-but his lawyer said he was getting very upset over all I was doing-I think this helped to keep him from doing anything stupid post moving out.

"Drug-related illegal activity" means the illegal manufacture, sale, distribution, purchase, use or possession with intent to manufacture, sell, distribute, or use of a controlled substance (as defined in Section 102 or the Controlled Substance Act [21 U.S.C. 802]) or possession of drug paraphernalia. b.

I notified local law enforcement about what I had seen and they made several arrest at the property, including the tenants, and found drug paraphernalia on site.