Looks like I'm going to have to get Ms Kitty on the phone and have a talkin' to her!
Alright - so it seems we have ruled out as many possibilities that can be thought of right now - except for two.
One would be you have been such a good owner you spoiled her rotten by escorting her to the litterbox - and now she expects it! This would mean (and you probably won't want to hear this) she will have to be isolated to a very small area - just her, a litter box, food/water, toys and that's about it. Reducing ther area of access reduces the possibility of pee'ing in inappropriate areas. This would have to be done until she consistently show's you she can use her litterbox and nothing else. But before you give her free access to the house, you would then increase the size of the area and repeat - until you have complete confidence she can be trusted.
The second possibility is a health issue. I realize she does not show any signs, symptoms, left you a note, or has done anything for you to believe there's something medical going on - yet, cats are very good at hiding illness and honestly, you can't rule it out until she has been thoroughly examined. If this option isn't taken and she does have an illness - we could be talking about this for many months to come!
Those our my thoughts as of tonight. Your thoughts now? (in just so happens I'm flying into Dallas tomorrow night - tell her if she doesn't straighten out, I'm going to come and toilet paper her cat room! )