Your mother has reported to you now a second time that someone is damaging her car, and you discover that indeed she has a dent on her car.  When you investigate you see that she is hitting the side of the house with her on her way past the house to the street. Not only is she repeatedly causing damage to her car, she thinks that someone else is doing it.

So, What is happening?

In this situation you have a parent who is likely having cognitive issues or sensory problems, be it sight hearing, or both.  Another possibility is that your parent is driving while under the influence.

When parents start having small accidents with no knowledge of what happened it is time for gentle action.  This is not a simple accident like scrapping the side of the garage door frame or hitting a "big wheel" parked a little too close to the back of your car  when backing out (my personal story when the kids were little) while being fully aware that it happened.  I am talking about someone who does not know they had an accident.  When you hit anything you can feel it, unless maybe you are driving an armored vehicle.

What needs to happen is to walk with your mother first to the car and then to the house where she hit it.  Then make a cup of tea, sit and listen. 

In this situation you have a parent who is driving impaired in some way, because when they have an accident she either cannot remember or was never aware that it happened.  Either way it is a problem that needs to be addressed.

How to Approach the Situation

I suggest these words: 

“Mom, you have always been a very safe driver.  Your minor accidents of hitting the house and the fact you are not remembering them is cause for concern. We need to look into this further.”  

You are likely to get denial next.  Maybe a change of subject will be attempted by your mother.  As a mother, I am sure I would try to change the topic of conversation in such a potentially embarrassing moment, but persist you must.  Hitting a small child could be next, and you want to protect your family from that type of trauma.  Feel free to mention the worst case scenario to your mother.

Now comes the hard part, go for a drive with mom at the wheel.  Watch how mom drives, the speed, the depth perception, and the judgments she makes.  Accompany her to medical appointments if she will allow you to do so.  Call your local DMV for advice, as they see this daily. 
 
Do remember that driving is a rite of passage and very difficult to give up.  If it is time for alternate transportation is prepared to help find solutions.