Sometimes the best way to hide anything is for it to be too obvious. Think of magicians, illusionists, on stage. A securely fastened key box could be normally on view but be hidden behind a hinged or lift off house number plate or house name plate. There is also the two stage option whereby a push button key box has a key to another box which has the door key. All push button devices are vulnerable unless code is changed frequently the open sesame buttons become shiny. Also applies to touch screens of tablets and phones.