I bought these coin-sized magnets for some project and didn’t use them.
They were just stetting away from anything magnetic-sensitive, till one day I was trying to figure out where to hide a key and my brain did a 2+2 and came out with sticking one of the magnets to the key and just find a suitable place.
It worked better than I expected. And the funny part is that you are not subjected to place the key where there is metal, but you can add a metal “location” by simply adding a wood screw to any part of the house you can think of for hiding the key, just make sure it is flush with the surface, your fingers will thank you. Do you want to rotate locations? Fine. Select several places, install screws and you are done.
There is also the obvious way of just selecting a hidden spot and affix the magnet to it: it also works. I am using both ways just fine.
And unless you are way too obvious, it would be very hard for somebody to find the key easily or at all.