Do you mean this picture?
I think one fan asked Himaruya (Hidekaz Himaruya, the author of Hetalia, if you didn't know C:) how Netherlands would look with hair down (or with wet hair, I'm not sure) and so Himaruya drew it and posted it on his blog.
I found the blog post! http://himaruya.blog61.fc2.com/blog-entry-951.html
His scarf is also just his scarf, he's been shown with it even with his hair up.