Will a person in late stages of dementia repeat a phrase unknowingly all day?

I used to be the head nurse on an Alzheimer's unit. My answer is yes, definitely...we had patients that repeated the exact same thing all day.

Maybe she senses something is wrong but simply can't recognize or communicate what it is. This happens with my mother.

This is a common thing from what I have read. I think it is more common with certain types of dementia. The one person that I knew who did that seemed to get comfort from having a sitter go and sit with her. Just having someone by her, talking, reading or just holding her hand seemed to stop the calls for help, at least for the moment. The patient was not able to engage in any conversation, except she would call out for help.

Frances, here is a list of stages for Alzheimer's/Dementia that will be helpful for you to have to know what might be coming next. https://www.agingcare.com/articles/Stages-of-Alzheimers-disease-118964.htm

There was one older lady at my Mom's nursing home who was always calling out "help me" even when there wasn't anything that she needed. Chances are your Mom is frighten but doesn't know why. Check with her doctor to see if there might be some meds that could calm her down... it will be a balancing act to get the right med and the right dosage.