When you’re at Estonia and walk around at night you’re required by law to wear cat eyes. A special situation I never had before and they tell you also where to wear them: at knees height on the right side of your body. It is necessary that they are visible from back and front. I learned about this from the travel advice of the Federal Foreign Office of Germany.
But to be honest you won’t typically see too much darkness. When I was at Tallinn the sunrise was at 4am and the sundown at 10:30pm. If you want to adhere to the law you can buy cat eyes at different places, typically museum stores have branded versions in stock. In the end wearing a reflector isn’t the worst idea – so why not take one home?