Not sure of this will work, or how well, on the floor boards. A tester area on the finish would be worth considering, but have you tried vinegar? Many years ago we had a young persian puss who decided to pee anywhere but in his tray. We received the advice that vinegar neutralised the urine smell. We washed the affected areas with vinegar each time puss had an accident (or was it deliberate? ) which kept the smell out of carpets, clothing etc. i've used vinegar now for years to clean up, especially the cat tray, and find we have no lingering odour. I suppose you would be refinishing the floors so you could be quite liberal with the application. On the plus side - it's a cheap solution!

Is that baking soda in your pic? Yes, a great odour absorber. Vinegar, as suggested by kazzh, might work as well. I'm thinking it's cat urine too. Potent and lasting. What is your ultimate plan for the flooring? Do you want to get back to the original hardwood or keep the new over the old? Perhaps there's an odour blocking flooring layer that exists the same way there's odour-blocking paint primer?

Yes, that's baking soda in the photo. @kazzh did the floor go into your subfloor or was it surface level? @tartanmeup We'd ideally like to keep the top layer of floors in tact, but it's increasingly looking like that's not going to be an option because we have to get to the source of the smell somehow/

The unit we were in at the time was carpet over concrete slab, so am unable to help with how far the urine soaked into the floor. what I can say is that puss peed on a back pack all those years ago (20 + yrs, and he really was a gorgeous cream Persian!), I washed the back pack in vinegar and still have it and no odour! Being liberal and mopping with vinegar will hopefully be effective for you as it sounds like the boards have held the urine for quite a while.

If the smell is unbearable after pulling one plank then I will hazard a guess the stain is either rather NEW or it has obtained LONG TERM multiple soaks. Either way, the former homeowner KNOWINGLY covered it up.

Here's my issue. The homeowner knew of the damage and, depending on how long ago your purchased the home, could be legally and financially responsible for the 'fix'. I don't know your state nor how long ago you purchased....but talking with your Realtor will help you understand what type of recourse you have.

The 'fix' = rip out all the current hardwood to expose the layer underneath. Then you will need to investigate (ie. rip out a few planks in the worst places) to see how much the subfloor is damaged. The possibility of ripping out the old hardwood AND the subfloor is high. And that gets expensive.

That's why you want to find out what your recourse is with the purchase and who is responsible for paying for the damage.