​Small reflective square tacked on with nail showed up on my fence post. What is it?

We found this on our fence this morning. The driveway is ours, so it’s on the outside of our fence. It is reflective and the dot in the middle is a nail. There is no reason for this post to be more visible, it is set back off the sidewalk by several feet. I don’t necessarily think it’s anything nefarious but I’m curious as to what it is and why is it there?