Peeing on the Carpet

We have had Guinness for about 3 weeks now. He is a 3month old Black Lab. Recently he has been going to the sliding glass doors to let him out, but just before I get there he starts to pee. I don't get it. Last week he was going to the sliding glass doors puting his paw on it and waiting for us to open the door so he can go pee. I can't understand why all of a sudden he goes by the sliding glass door and pees right in front of it. Last night was the second time he did it. It is so fustrating

He must smell it on the carpet. Get Nature's Miracle and clean the rug really good. Vinegar works too. They go where they smell their pee.

Nature's Miracle is supposed to be really good. There is a phone number on the bottle that you can call and they will tell you the proper way to use the product. They told me to totally wet the area my puppy is wetting (tile and wood floors) and let it dry naturally. It is the dried on chemical that takes away the scent. Carpeting is probably the hardest of all to clean. The urine soaks through to the backing so you need to be sure the cleanser reaches that also.
I read that dogs have 1,000 times better sense of smell than people do. No wonder we have such a hard time training them.
Good luck. Believe me, we are in the same boat and it's no fun constantly cleaning up, especially 5 or 6 in the morning.
Let me know if you get any good tips.