I'm unsure how new this sign is, but I found it for the first time today, it can be located on the Sydney Building, just behind Platform 3. I've walked on the other side of this wall millions of time & only just noticed it.
(https://www.actbus.net/forum/proxy.php?request=http%3A%2F%2Fi611.photobucket.com%2Falbums%2Ftt193%2FBus400%2FMiscellaneous%2FIMG_0048_crop.jpg&hash=b9143c2794ee6aaf6b9d5f409dbf663320e3de96)
Interesting.. I've never noticed that there either...
Very new, they are being placed up all around the building.
Speaking of which, those buildings need a good touch up prior to our 2013 celebrations......