Hi there, I have the same problem with my 6 year old Bichon Frise. He is fixed and everything and even if he just went out, sometimes he marks around the house. The vet said to test him for urinary track infections or for any other disease, but he doesn't have anything, he is just one of those dogs who has a problem with marking around the house! I just keep an eye on him and when I am not home I put a little diaper on him. (he hates the diaper but that is the only solution!)
I hope this helps
