There's water that is somehow damaging the hardwood floor by a back sliding glass door, but we can't figure out the source.When I've had other contractors check, we've found no moisture in the walls or even in the ceiling of the basement below -- the moisture is only registering on the floor.There are also no pipes in the area.My only guess is that this might be coming from some kind of roof leak that is just passing the walls and collecting on the ground in the walls.At the end of the day, I need someone who can help figure this out and can help us fix the issue.Got any suggestions?

Generally illegal activity within the unit is grounds for eviction... all my units are also non-smoking so if they are smoking an illegal substance, they're violating twice over.Frankly, I'd get them out... the longer they stay, the worse that smell is going to get and the more it's going to penetrate every pore of the residence.  

Then add to the lease a statement something like 'occupants are not allowed to tamper with or disable moisture alarms' in addition to the usual language about notification of leaks.

Seamless roofing= no way for water to penetrate through. 

If it is rain water penetrating through the roof and making its way to the interior space the sheetrock, insulation and framing may need replacing as well.

Make sure the basement doesn't have any moisture issues (ours did, and we didn't find out until after we owned it). 

Find your leak / moisture first and fix.  

The exterior walls have moisture at the bottom that led to mildew on the bottom part of the sheet rock.

Ideally, some of those super powerful fans that blow right along the floor.Get a moisture meter off amazon and monitor the water content of the framing. 

And all our old basements have their own problems, like moisture and mold.