Or wet it to a paste, and dip in cotton balls that can be placed in out of the way places..Of course, getting the food sources cleaned up and sealed away is the first step and reminding the tenant of the attractants may be the key.

I would recommend swapping out smoke and CO alarms with the newer 10-year sealed ones.

Unless this is an A+ rental, the toilet itself should have been $75-$150, and add in a few dollars for a new wax seal. 

It will adhere well to the oil-based primer and seal in the smell (as much as possible). 

Otherwise, consider LVP planking sheet vinyl to create a seal and update the look/feel on the cheap. 

Or is it because the doorway is not sealed properly and somehow the rain water is getting into only that closet?

Cheap labor often requires more oversight.A tenant turnover that has taken good care of the unit is nearly $1k (patch pin holes and touch up paint, steam clean carpet, seal grout/granite, toilet flaps, clean drains, misc).  

It's not just the porosity and sealing issues, well-chosen granite is not nearly as vulnerable to that as some of the trendier lighter more open grained granites that people felt compelled to use by their designers. 

Oh, and better windows, too - sealed everything up and whatever the best double-paned windows were way back then. 

.) - whose great idea was it to create pipe threads that need either tape or pipe dope to actually seal?