Those "realtors locks" and similar key-safes are often vulnerable to shimming or other bypass techniques. Or just a sharp blow with a nearby hard object due to being made of poor quality material...
One that I see a lot is the "Master" brand ones, with a 4 digit combination. Not only can they be smashed open easily, but they are also vulnerable to a manipulation attack that allows obtaining the combination within a couple of minutes.
As for the "fake rocks" and similar "concealment" devices, they are usually easy to pick out. They usually look out of place, and the area around them will show signs of activity. Some of them can be "spotted" easily at night by simply shining a bright flashlight around - the material they are made of doesn't reflect/absorb light the way the item they are meant to resemble will.
RE: Realtor Locks, Key Hiders, and Good Residential Security