Potty Trained Dog Peeing in House

So my sweet Great Pyr/Lab mix is potty trained for all intents and purposes. She and our male boxer mix have fenced in 1/2 acre with a doggie door for free access in and out 24/7.

Boxer does not have accidents after potty training as a puppy. He’s in and out the doggie door as needed. Our Pyr is a rescue that we adopted as a 1yo about a year ago. She was surrendered because she was too big for her previous families small children.

At any rate, she also uses the door to go in and out as she pleases but we have an intermittent problem with her peeing in the house. We’ve had her tested for a UTI a couple times to check and she was never positive. Also, it doesn’t seem to tie to stress. She’s a confident dog and doesn’t seem to get nervous. It does happen occasionally when we go somewhere but also when we’re home. She’ll sneak away to another room while we’re watching TV in the evening. Other times like when we’re putting the kids to bed. She knows she’s wrong because on the rare occasion we catch her in the act she stops immediately and leaves.

Any thoughts are appreciated. Thanks!!