Try to get a free inspection from mold assessors, structure companies, crawl space encapsulators, etc. paying particular attention to the wood moisture content under there.

The unit has also been tested for environmental issues and moisture was below "dry levels" and indoor air quality levels were okay though CO2 levels were just above guidelines in some rooms.The tenant is known to use a lot of candles, we're wondering if this could cause this.Does anyone have any ideas?

That'll alleviate the moisture issue.

They find that the water on the ceiling was either due to the moisture/humidity from taking a shower or from just water splashing.  

Many landlords don't do what is necessary to mitigate the risk of mold and moisture in the home.

Basically I am wondering if anyone has ever had something happen to a property that created an issue for the tenant to stay there until repairs were made. The unit is roughly 500 sq ft and has two adults & a ...

not sure if i mentioned this one... a few months ago the tenant called and said there is some moisture/water dripping from the kitchen cabinet under the sink.i went there and saw the entire inside of the cabinet is wet. it's been dripping for some time. i asked her why she did not call me earlier and she said, "I never use this cabinet". when i looked around, half of the kitchen cabinets were empty.

Did you check her ceiling with a moisture meter?  

Brick and mortar need to breath, especially if any kind of moisture gets in because it will be trapped and degrade the mortar.

I would pass.....Couple of issues/concerns...... he's kind of running a business out of your unit, which is never a good idea to let tenant do...... can start small and innocent but then keep growing and expanding.....Growing weed takes a decent amount of electricity, heat and moisture.....all can be bad for a property if not contained.