**For those that don't know a MacGuffin is a plot device in movies. Examples are the Maltese Falcon, anything Indiana Jones was after, and Charles Foster Kane's sled.
So, you know, your MacGuffin, is what people want.
Step 1: Hide That Treasure
Unscrew the plate to expose the outlet/switch and the remodel box.
Step 2: What You Need
A screwdriver or drill with appropriate bits
A container (here the ubiquitous altoids tin)
A thing to hide (here referred to as a MacGuffin)
String of some variety (I used some yarn I found)
A rubber band or some tape (not pictured because I forgot)
Step 3: Removing The Box
Step 4: Hiding
Step 5: Reinstall
If you want something that leaves absolutely no sign that its in the wall, then don't use the string. But make damn sure you use a steel box and what you put in it isn't all that heavy. That way you can tie a magnet to a string and lower it into the cavity to retrieve your treasure.
And if you die before you retrieve it then someone will have a very nice surprise years later when they go to remodel/demo the house.