Check this out:
Delusions (or strongly held false beliefs) are a common symptom for a person with dementia. They can take the form of paranoia, which makes the person feel threatened, even if there is no or little reason to feel this way. Dementia can make a person suspicious of the people around them.
A person experiencing a delusion may feel that they are being watched, or that someone is acting against them. They may jump to conclusions without much evidence.
For example, Because of dementia, the person may no longer be able to moderate extreme or irrational thoughts.
Sound like anybody we know?