Circlep, I couldn't have said it better. What a great description of marking behavior for the dog owner. I have lived with male & female mini-pins in the house for the past 15 yrs. I never neutered my male because I used him for stud. It doesn't matter how much they are outside, the males will mark in the house. It will never stop no matter what you do. Cinch, I would do as she says, get him fixed asap and get the urinary system checked out.
Taking them outside more will help with the dogs that don't like to go outside, especially if it is cold and wet. That last trip outside before you go to bed helps a lot. In bad weather minipins refuse to go outside. You have to physically go outside with them. Not up to the trip yourself? Then opt for pee pads. If I were trying to get control of a pee problem I would avoid the pee pads to start out with. You want to impress on them the habit of holding and then going outside. Crates really do work for when you are gone and developes that habit of holding and then going outside.
I would never buy another male puppy without neutering it just because of the marking. I am so tired of dog pee. When King goes to Doggie Heaven, there won't be any more.