Please advise:

I am renting from someone with hardwood floors. She told me heart of pine with 3 coats of poly. I believe we are the first renters on these floors, though some parts look new and some look older (I do not know much about hardwood floors as I've always lived on carpet).

We have been here 2 weeks, and I just noticed this spot where it looks like something has through the coating to the wood. We don't know if it was already there, or if it's possible that we didn't catch the dog doing a tiny mark there. Could a tiny amount of pet urine dissolve the polyurethane? Or did something else cause this? I have tried to be vigilant with the dog. He does not usually mark, but it does rarely happen.

https://postimg.org/image/c57jwpp33/

This looks like a fresh pet stain. If the urine has lifted the finish it will be a different texture when you rub your finger over it. Watch out under your water bowl, too.
What a beautiful floor! I hope you have floor protectors under ALL your furniture.
Try to take your pet for more frequent walks.