I saw it today on a spot the husband tried cleaning but they weren't sure what was spilled.
I started with a dry cleaning solvent to cut residues and removed some soil but the ring became more pronounced so I doused it with vol 40 and it disappeared. I cleaned it good and left a fan on it.
I hope it doesn't come back.
That should end the issue with the customer.
If it was worth the time and effort, multiple sub-surface extractions and an aggressive cleaning of the entire room, and application of an anti-wicking agent might eliminate it for good, but depending on the amount of soil and urine in the carpet, a surface touch up like that is probably for the best.
I'm to the point of spraying an anti-wicking agent on all urine subsurface extractions, as wicking and watermarking are pretty common now, especially with polyester and triexta.
(To be clear, I'm not in the business, but I am a landlord who allows pets, and I have a friends and family who have pets so.....)