This must be one of the simplest and best hiding places going.
I keep a spare set of house keys inside an old caulking gun tube, hanging in plain sight in my garden shed.
No special tools or skills required...ANYONE CAN DO THIS!
Step 1: Keep an Old Dried Up Tube of Caulk or Silicone
Step 2: Pull the Plunger of the Gun Back to Release the Caulking Tube.
Step 3: Insert Your Keys or Other Valuables
Step 4: Reinsert the Plunger and Push Down to Secure the Caulking Tube
Step 5: Now "Hide" the Caulking Gun in Plain Sight
Just hang it back with your other tools, or just leave it lying on your incredibly tidy workbench!
Nobody will ever think to look inside an old dried up tube of caulking.