House Trained Dog Peeing Inside?

I have a 7 year old Boston Terrier who has been house trained since she was a puppy. She has always had a dog door and does use it frequently. However, every time we leave the house she pees somewhere in the house. She has a few favorite places she goes, but she also, occasionally, will go right in front of the door that we leave and enter through. One time we gave her a shower and then had to leave shortly after that and she peed in the shower. Sometimes she'll go right in front of the dog door.

She's starting to branch out into different areas of the house now and it's getting frustrating.
We are military and we move every few years which I know is stressful for her, but we've lived in this house for well over a year.
I keep reading that they don't do it out of spite, but that's hard to believe when she goes in front of the door or in the shower after she's been bathed.
Any advice would be greatly appreciated.

By Megan J.