Ender 3 question - where did this come from?

[tohm]

I recently purchased the Ender 3... still have a lot to learn! Was printing a box for hours and when I came back to the printer I found a small rubber cover inside the box I was printing. The box was really messed up and didn't come out the way it should but the problem I have is... I don't know where this rubber cover came from and where it goes. It's about the size of a quarter and has CR engraved on one size. Can someone tell me where this cover came from? THANK YOU!!!!