I have found a way to hide objects in a Google Jamboard Jam. Create mystery spots in the Jam so students pull out hidden objects.
Pen Stays on Top
The key to a mystery spot is understanding that in a Google Jamboard Jam the Pen tool is always on top of objects. So if you draw on the frame any images, textboxes, or shapes will appear UNDER the pen.
Add More Pen
The trick to the mystery spot is to scribble a lot of pen onto the frame so that it creates a spot.
Use a White Pen with a White Background
Tip to either use a white pen on a white background or a black pen on a black background. This will cause your mystery spot to be invisible on the frame. My trick for creating a mystery spot is to temporarily set a background color so I can see that I have covered the spot entirely. I then switch the background to a white background (or black) so the spots disappear onto the frame.
Hide Items on the Frame
Drag images, textboxes, and Sticky Notes under the mystery spot. They will disappear on the frame.
Consider the Layers
Items such as Sticky Notes, images, and textboxes layer. They are layered in the order that I added them to the frame. If I drag these items into the mystery spot, they will disappear on the frame and be hidden. Stacking them under the mystery spot they will be dragged out of the mystery spot in the reverse order of how you added them to the frame. Consider the order you want items to be revealed from behind the mystery spot by reordering the items before hiding them.