My mother is 81. She lives alone. For many years she has been hearing knocking on her house outside of her bedroom. It happens around 2-3am, and happens a couple times a week. She was convinced it was her next door neighbor. She since has moved across town and is hearing it again. She is convinced it's a new neighbor, but doesn't see the correlation. She installed cameras on the outside of her house in both places and nothing is there. Could this be a symptom of a health related problem?
Or wildlife that roam around at night. Since it is around the same time during the night, wildlife has a set pathway and set schedule.
Are you noticing ANY other odd, strange, things that might be an indication of dementia? I am not talking about recent things only think back over the last year...has she been lost, has she misplaced items (heaven help all of us if this is an indication) has she been confused about a time, a date, an appointment? Has she called and asked for simple things to be done for her, or an excuse that something was not done? (my Husband was always doing things around the house and when I asked about something he said.."when I feel stronger" that was not a response that he would have normally given)
Have you noticed out of date food in the fridge, has she been skipping meals or eating more than she should?
There are so many little things that can be brushed off as nothing but when you add them all up they really mean something!
So it could be a sign of dementia, other hallucination, could be caused by medications, UTI, or it could be the water pipes, the sump pump kicking on (mine goes on when the water softener cycles about 2 am)
Best way to find out is to get her to the doctor. As part of a regular exam she is or should be getting a MME or you can ask for a referral to a Neurologist or a NeuroPsychologist.
Even if you are having a hard time identifying the cause of the knocking, it could still be a real sound. My mother complains about hearing voices outside the house sometimes and I was convinced they must just be in her head... until the night I went to the kitchen about 3:00a and saw the next door neighbors out on their front porch with their dogs running in the yard. Mom's bedroom is on the side of out house next to those neighbors. They work 2nd shift and will enjoy their porch in good weather and play catch with the dogs, both before and after work. Almost every time Mom would say she was hearing voices, I could look out the window and see the neighbors out on their porch or in their yard or evidence they had recently been there (tea glass or coffee cup on porch table).