My 3 1/2 year old dog is housebroken but is sneaking off to pee in the house. Can you help?

I have a question regarding my three and half year old Belgian Malinois. I got her when she was about two and a half years old wandering the streets. When I first brought her home over a year ago, she would urinate in my kids' bedrooms and my closet. I assumed house breaking issues so I purchased your eBook on housebreaking and followed the steps there several times, always having to start over. I recently got married and that has made the problem worse. My wife found that she had urinated and defecated on my wife's side of the bed. Now she will sneak off and urinate in our closet. We have tethered her to us so she could wander off and even resorted to leaving her in the crate unless she goes outside. My wife has seen her peaking around the corner in the hall like she was checking to see if she could pee. She seems to be housebroken but she is sneaking off to do this. We keep her crated, but sometimes the kids forget and then she sneaks off and pees. Any help would be great. Thanks.