If we see a dog that is housetrained, not 'sorta kinda' but truly housetrained, a sudden change presents itself, the first thing to rule out is a bladder/urinary infection. Normally with an infection, you would also see an increase of water intake, increase in urinating; straining or inability to empty the bladder; and sometimes blood tinged urine.
Yet, if she is only having these accidents that last pottybreak at night, it very well could be a behavior problem and not a medical one. By your wording, it appears you are with her that last outing - and you need to be to make sure she actually goes. Sometimes dogs "forget" what they need to do at this time - it's like their last 'hurrah!' to explore the world before bedtime. If she is not urinating at this time, what is she doing? If she appears timid, could something outside during that hour create a fear in her? Or did something happen in the past during this time that frightened her. An example I experienced with a clients dog - once when the dog was outside a car backfired - the dog wouldn't go outside on it's own for a long time! Is she being 'busy' and not focusing on what she should be doing? In this case, take her to the same spot, on leash, and wait...and wait...and wait. Have a treat ready to give to her when she does finally go. While you are waiting she should not get any attention from you or anyone else - business first, and then play and cuddles.
If the above are not the situation, then I would make an appt with the vet for a urinalysis.
I hope you have found this answer to be informative and satisfactory - all the best,
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Oliver's Mom
