It may be tempting to hide a key within the devices below, but I highly recommend you do not do it. No matter how clever you think you are, the hidden key will be found. You can put the key in a fake rock, under a mat, above the door, in a magnetic key holder for your car, or in a realtor lock as the one pictured below. All of them are a very bad idea however.
First of all, let me explain a little more about the realtor locks above.
They are like having a big "Steal My Stuff" sign on your house. Never use them where they can be seen. Thieves see them, assume the house is empty, and then come back later. If it is empty, what on Earth would they steal you ask? They take the appliances, copper piping, and more. Even an empty house has a lot of valuable items within it.
See those wheels with numbers on them? You can push them aside with a tool to see the notches for the combination. Make sure you never use that type of lock for anything of value. They are practically worthless for security if the thief knows what they are doing.
The above image is the classic. Everyone thinks they are so clever, right? Who would think to look for such a thing? They don't actually have to look even if it is hidden well. I've seen these key holders that look like piles of dog poop even. The people buying them are missing the point, and the point is that your neighbors or a random person walking by will eventually see you use it.
Worse, if you have a lot to steal or are considered a "high value target," a thief will setup a time lapsed camera to watch your property. With it they will get your schedule, all of your habits, and where you hide your keys. They don't need to even be there. They just set the camera up, let it run, come back to get it, and then process the data.
I have very expensive security locks installed on my home. They were hundreds of dollars each. Leaving a key in my yard to make it easy to bypass that layer of high security wouldn't make any sense at all. It's an entirely different topic to post about, but hopefully you have at least decent locks installed on your home.
The default "builder's grade" locks are terrible, and everyone should consider purchasing at least mid-grade locks to avoid easy lock picking, bump keying, and other commonly used tactics by thieves. If you need assistance with selecting locks for your home, please let me know. I will gladly assist you as my time allows.
One of my brother's, who is a master locksmith, can assist you as well. His handle on here is @ligneous!
Finally, if you need to leave a spare key somewhere, leave it with a trusted neighbor. That is usually the best strategy.
