Hello Sue, and welcome. Delusions can certainly be a symptom with dementia, as can paranoia and both visual and auditory hallucinations. What your mum seems to have would come more under a delusion (a firm belief that something is so, even though she neither sees or hears anything to back up her belief) whereas with hallucinations, the person believes whatever it is to be so, because they are seeing or hearing the "evidence". Although, sometimes the person knows that they are hallucinating, and that what they are seeing is not real. However, it doesn't necessarily follow that what your mum is experiencing is caused by dementia.
I would mention the situation to your mum's doctor, who could call her in for a "routine check up." and take it from there. If it should turn out that your mum is developing Alzheimer's Disease, or one of the other dementias, then perhaps it would be appropriate for the doctor to start her on one of the medications that can, in the early stages, slow the progression of the illness. And besides, forewarned is forearmed. What is a benign, non threatening delusion today could turn in a twinkling to something that is terrifying for your mum. At least you would have her doctor alerted.