Since our old cat was ill and started soiling indoors we have struggled to locate the source of a lingering odour. Bit of googling later I decided to buy a blacklight/UV torch.
I read various good/bad reviews, read up on the required light spectrum for pet mess and made a choice.
Well to say I was stunned is an understatement! Here's what an old (we're talking 6 months or more) cat wee stain looks like under UV. Bear in mind that the pic doesn't do it justice, the stains are probably twice or more bright than they show in the picture.
If you find yourself in the same pickle then I can heartily recommend it. Coupled with some stuff called PetWeeAway (which really does work) I can finally get rid of the stinky remnants.
Torch wasn't cheap (but not dear either) but research suggested cheap=poor in this field so I plunged. Happy to share link to the one I got if anyone's interested.
Cheers
Mark