I have a miniture schnauzer (male) that we have owned for over 10 years, he is the perfect dog, very calm, wonderful dog. He has NEVER used the restroom in my house until lately. The last couple of weeks, he has been using my closets, wicker baskets of my plants, doorways, etc. I just don't understand what is causing this? Any advice?

Older dogs will develop diseases that need to be looked at by a vet.

He could be becoming forgetful like some old dogs do, or he could be telling you something is wrong.

If this where my dog, and there was a sudden change in behavior like this, I would take him into the vet. Thinks like diabetes and kidney failure are a good possibility with dogs who start urinating in the home.

Dogs, like many people, start to lose the ability to hold it in.
I take my old dog out a lot more than I used to. Sometimes she can't make it through the night so I have paper towels on the bathroom floor.
Close doors of rooms you aren't using.
There's also the possibility of UTI.