Monday Mystery

On Saturday, I went downtown to pick up Mindy. As I was waiting for her, I noticed this sign.

What does this even mean? It seems to me that there should be some punctuation for it to make sense. It could be:

Attention!

Poles behind, back with caution. 

Translation: Polish people are behind your car, so back up with caution.

I've tried it every which way and still cannot figure out what this means. It's a mystery to me!