If you've got secret goodies to secretly secret away, then this might be for you. It's cheap, it's easy, and only requires that you have an electrical remodel (old work) box somewhere in your home. And if you don't, you could remove a switch or electric outlet to put one in.
**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
If you're lucky enough to have an electrical outlet or switch that is in a remodel box (or "old work" box) you can easily access the hidden crevices of your home. These spaces are ideal for concealing money, jewels, precious metals, or green herbs that you smoke that are now becoming legal in states that you probably don't live in.
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
There are usually two screws to install/remove electrical remodel boxes. The one pictured has two on opposite corners. Unscrew them and carefully remove the box from the drywall to expose the cavity.
Step 4: Hiding
Put your MacGuffin in the tin and wrap a rubber band or some tape around it. If you're planning on leaving this in place a long time you might want to punch some holes in the box to attach the string. I didn't plan on this being there long enough for the rubber to degrade so I tied my string on to the rubber band and lowered it in. Once it is in place on the 2x4 at the bottom of the wall I lightly wrap the string around the electric wires. You shouldn't be nervous about such things. Just keep the string loose and it'll never have a chance to cut through the wire's insulation.
Step 5: Reinstall
Reinstall electric box and cover it up with the electrical plate. Clean up the area of any drywall dust that might give away that the outlet has been messed with.
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.