Shippo is House-broken

Shippo is completely house-broken and is trained to never pee or poop in the house, until he is in a house that has a lot of pee stained in the carpets. I'm not sure if he gets confused or decides that he needs to mark territory (he was neutered at a young age) but he will spray on the stained parts of the carpet. I will need to watch out for this next time I go visit someone who has stained carpets.