Important Disclaimer: Use This Trick Carefully
This mirror-bag trick only makes sense when your car is parked. Do not drive with a bag over any mirror. Your side mirrors are there because you need them to change lanes, reverse, spot cyclists, notice pedestrians, and understand what is happening around the car. Blocking that view is dangerous and may also break local road rules.
You should also think about the message the bag might send. In some places, a covered mirror could make your car look unattended, unusual, or easier to notice. If you are traveling alone, especially at night, do not use the trick in a way that makes you feel more exposed. Park in a lit area, keep valuables hidden, lock the car, and trust your instincts.
Finally, use the right material. A loose plastic bag can blow away and become litter. A wet or gritty bag can rub against paint or mirror trim. Secure it gently, remove it completely, and consider proper mirror covers if you do this often. The bag is not a must-do safety rule. It is a small parked-car trick that works best with common sense and caution. When in doubt, skip it.