You may have a water line in the wall that would involve both the crawl space and the odor wall that has been either leaking or seeping for quite some time. 

it has organic vapor relief and will help masking the odor a little.  1) open the blinds if any and let as much sunlight in a possible.2) open all windows, let it vent for a few hours before the cleanup.  

Originally posted by @Theodore Arzt: I just rented out my first multi family.

Yes, a blacklight will illuminate urine, but it won't do a thing to eliminate the odor, which is probably what she really wants to know . . .   ;-)

Get a quote to have the odor neutralized and with hold that amount from the deposit.

@Theodore RiveraThank you!

Avoid cats because the odor is very difficult to get rid of if the Tenant doesn't care for them properly.

We had to take the drywall (walls) out to the studs & the insulation was also removed & replaced before we could get rid of the odor d'fume .

@Theodore Ezzedine everyone feels this way about first deal.

Additionally it doesn't have the odor for guests and is supposed to be better for skin (perhaps a slightly better pool user experience).