Turning on the fan will also help eliminate bathroom odors and odors from harmful bathroom cleaning chemicals.Pro tip: Add cleaning the exhaust fan on your cleaning crew’s checklist during turnover.

If they were smoking outside, the odor would quickly dissipate and would not permeate the building envelope and infuse into the other units. 

Maybe they've been burned by landlords in the past and are being very careful.You can legally reject them based on personality, facial tattoos, body odor, or other factors, but you better make sure you can prove that it wasn't based on any protected class like race, religion, familial status, etc.

The rental had ratty furniture, a record player prominently featured in the marketing but it had no speakers, missing basics items in the kitchen, lots of drunks in the concrete hallways that reverberated right into the bedroom at 2am, a strong odor of urine outside, and more.We have super hosts in this forum that have hundreds of 5-star reviews and are masters of the craft, but I look at their rentals and see they don't have any artwork on the walls, decor is basic, they do nothing to make the guest feel welcome, etc. 

The odors are not always easy to eliminate within the normal apartment turnover routine.

On top of that you have idiots who engage in bid wars and pay retail for a house that has never been inspected (and those are the bidders who will cry foul when they discover totally trashed and destroyed house that will require a full gut job to rehab). 

A restaurant downstairs with odor, and noise till 3:00am is unsuitable for residential tenants.

Now there was no mention of the engineered plans requirement for the 12+ requirement to my recollection, so I do call foul there. 

No harm, no foul. 

The bigger it grows the larger the potential for someone to cry foul, and the hammer comes down.