Dog peeing in house

[Roadie]

Dakota, my shelter pup that I've have for about 4 years now, came to us when she was about 1.5 years old. She was housetrained already, and as far as dogs go, she's a dream. She doesn't get on furniture, doesn't chew things up, rarely barks, tolerates the cats, doesn't jump on people, and generally imitates a floor rug. Since she's lived with us, she has always slept inside, except the occasional skunk incident or other weird exception.

A little over a month ago, while I was out of town, she had accidents - both pooping and peeing - in the living room at night when DH was sleeping, 2 nights in a row. For the next few nights, she slept outside. She would ordinarily protest this, but the weather was nice, and my MIL's dog was visiting so she was enjoying company. When MIL's dog went home, Dakota came back in, was good for one night and had an accident the next. She spent the next week sleeping outside.

I brought her in after that, and she's been fine since then - until (I think) last night. It was either last night or the night before - I wasn't home the night before, but DH may not have noticed since there was no stinky poo this time.

So, Dakota is about to become an outside dog if I can't figure out what to do with her. Her days of wandering the house at night are over. Is she too old to crate train? How do you train a dog that is unimpressible? I mean she doesn't care for praise, nor treats, nor toys. She'd just prefer to be left alone. I've never been able to really "teach" her anything. She came house broken, and she knows how to shake, but other than that she's never done anything I had to break her of, nor has she really not done anything I needed her to do. On the rare times I've seen her doing something she shouldn't, I give her a stern "No!", and she stops. After a few times, she doesn't repeat it (she was a couch sleeper when she first moved in, but a few "No!"s seemed to cure her right quick).


ETA: There haven't been any environmental changes.

Help!