My dog poops on the floor?

She is 2 years old. She knows to go outside. She will stand at the door. I let her out for over an hour to roam the yard. We got for walks, we go to the park. Well, about 1-2 times a month, she sneaks off and poops either in my kid's room or in the hallway outside if his door is shut.. Mind you this isnt the pick up and throw away poop like outside. This is straight up slime.. Oh, and she is a GREAT DANE so imagine the pile.. not only is it there but then she drips it clear down the hall. Im at my wits end! Her diet hasnt changed, her environment hasnt changed. She is healthy and active. I don't understand why she does this! Or even how to discipline her. I put her in a crate that we use Strictly for her for time out and no other reason. Please help. Our carpets and my stress levels cant take it!