Shoot the banner above the fireplace - before:
This exposes a little wooden hatch in the wall. Now go to the picture on the wall and "use" it - it slides aside. Then use the control that was hidden behind the picture - this makes the wooden hatch open:
If you stand on the table, you'll see that there are two gold bars hidden in the nook which was behind the hatch - but how to get them?
It's tempting to try to jump onto the mantle of the fireplace (perhaps using a chair) but it's just too high. You can still grab the gold (on the fly) by placing a chair right in front of the fireplace and jumping from it toward the nook. This image above shows a good position for the chair:
As you do jump, aim toward the gold bars and hit your "use" control repeatedly. With a bit of perseverance, you'll be able to grab both bars. Each gives you a "treasure" credit, and taking the bar on the right credits you with a "secret".
Destroy the plan on the wall, and then the wall itself behind the plan, exposing a secret area.
To get the treasure (and be credited with finding the secret) you need to duck-jump into the aperture.