My husband has been helping my aunt with mowing the lawn and other tasks around the house since her husband passed away three years ago. She's been so appreciatative. In Novemeber, she accused him of storing something in her crawl space. This spring she accused him of stealing coins she had around the house. She's saying he's been doing this for awhile. She had all the locks on her home changed. She's saying there's evidence (things weren't where she left them), he's the only one with a key, etc.
It could be. However there are many causes of suspicious, possibly paranoid ideas and behavior. These can include bereavement, anxiety, depression, schizophrenia, medication side effects, a number of neurological diseases such as any number of different dementias, growths in the brain, drug abuse, kidney disease- the list goes on and on.
I agree with Carol that she needs to see her doctor to get an accurate diagnosis.
This certainly could be a sign. Please try to get her to a doctor for a checkup. New mediations can hold back some of the decline. She needs her medications checked, too, as many mediations can cause paranoia. Also, please read Christine Kennard's post Stages of Alzheimer's Disease: Caregiving Guidelines for more guidance.
My mom also exhibited symptoms of paranoia about what others were "doing to her". In our case, she believed my father was stealing from her. So I'd echo the two previous responses and suggest that you arrange to get your aunt to see a doctor to figure out why she is exhibiting this behavior.
Take care and keep us posted!