I take note of the BWJM's suggestion and I'll try with a short-wave UV lamp. I'll give you the results tomorrow since I don't have access to this lamp for now.
This square is interesting because it's not mentionned in the standard security features known in the public. But when our club had the speech about the counterfeiting and bogus notes from RCMP cops, they said that there was hidden security features in addition to what is publicly spread, for the simple reason that the officials can check these hidden features to ascertain the authenticity of notes. You cannot imitate what you don't know to exists...
And I believe that this square is the first of these hidden security features to be known. It's faintness is probably normal, and cannot be well seen with the usual UV detector used by the vendors and sellers in the commerce.
To see this square, when you light it with an UV lamp, make sure that there is not any direct normal light on it, preferably in a dark room (not necessarly a photograph's dark room, but you just leave the light off). It's color is the same on all denominations, except on the 5$ note, where this square is not printed.
And why we should limits on UV light? Infrared lamp can be used too...And maybe there is magnetic inks? or some other features ?