This square is the norm, and was put on all note of the Journey's serie and on a part on the Birds serie (mostly on the 20$). But this square is there for one reason. The printer would not put something absolutely useless on the notes...And I put one year ago an image of this square, which fluoresces feebly under a black light.
I don't know exactly what is it and I didn't get any positive answer when I asked the question to a RCMP agent who was invited to our coin club to explain the new polymer note last February. He wasn't aware of the existence of this square and wasn't able (or didn't wanted, I don't know) to tell us what is it.... Maybe the agent was instructed by his superiors to even not tell that this square is for...
All I was able to learn is there is some public security features (those one publicly known) and some secret (or confidential) ones. I guess that this square is one of these secret security features.