When I was in Amsterdam last December, I got an orange sticker on the back of my US passport. I don't remember whether it happened when I arrived or when I was departing, but I just saw it again and it made me curious. I couldn't find an answer here or on Google, so I'm wondering if any of you know what it's for.

Here is the sticker:

And the back of passport for scale:

You can't really read the text in the picture, but all it says is "Amsterdam Airport Schiphol".

I'm also wondering if that's something I need to leave on there. The sticker has triangular perforations, almost like what you'd find on some void stickers, so I'm wondering if will leave anything behind.