Ask a burglar where to find a spare key and they will name the same four or five spots without hesitation: under the mat, on top of the door frame, inside a fake plastic rock sitting next to the entrance, under a flowerpot by the door, in the mailbox. These are so well known that checking them takes seconds. If your spare key is in any of these spots, it is not hidden. It is on display.
Why location matters more than the container
A realistic fake rock is a good hiding tool. A realistic fake rock placed directly beside your front door is not, because the search pattern starts at the entrance and works outward. The single biggest improvement you can make to any outdoor hide is distance and context: move it away from the door, and place it somewhere it belongs.
Better spots, and what to hide there
A flower bed or shrub line away from the entrance is far better than a pot on the porch. A hollow stone safe matched to the existing landscaping, or a molded pinecone tucked under a shrub, blends into ground cover that nobody examines closely.
A working sprinkler head is one of the strongest disguises available, because irrigation components are functional infrastructure that draws zero attention even from someone deliberately searching a yard. A replica pop-up sprinkler pushed into the lawn, away from the direct path to the door, is about as close to invisible as an outdoor hide gets.
If you want an option with an actual lock rather than concealment alone, a lockable flowerpot safe with a real plant on top gives you a steel compartment in the base. Place it on a side patio or a back porch rather than directly by the entrance.
Placement rules that matter
Stay off the direct path from the street or driveway to the front door. That path gets the fastest and most thorough visual sweep. A side yard, the edge of a back patio, or a bed along a fence line all see far less scrutiny.
Match the object to what is already there. A grey stone in a yard full of red mulch looks placed. Choose a hide that fits the existing colors and materials in your specific yard, and set it among two or three real rocks or plants rather than alone.
Avoid disturbed zones. Mower paths, areas landscapers regularly work, and anywhere leaves get raked all move small objects around over a season. Pick a spot that stays untouched.
What not to do
Do not use the mailbox. It is federally protected in some countries for a reason unrelated to security, but practically it is also one of the first things checked. Do not use anything directly attached to the door frame or the door itself. And do not tell more people than necessary where the key is, since the safest hiding spot is worthless the moment three neighbors and a dog walker know about it.
A backup worth considering
If losing the only spare is a real risk, a keyless smart lock or a keypad deadbolt removes the problem entirely. But for a low-cost, no-installation option, a well-placed and well-chosen outdoor diversion safe is still one of the most practical solutions available, provided it goes somewhere better than the mat.