It can also be an eye issue...........macular degeneration had my mother seeing things that weren't there, too. Like constant raining outside her window, for example. So maybe have her eyes tested also. You do, however, say in your profile that your mother suffers from Alzheimer's/dementia. If so, it's probably a manifestation of that disease. Run it by her PCP to see if thinks a medication may help her.
MsVicki, I agree with JoAnn's posting. I remember when my Dad was seeing ants on the wall and in his food. Sure enough, it was an Urinary Tract Infection.
Good luck!