If you leave the key in the door and you have second key you will not be able to unlock the door
If the door isn't double locked they are easy to slip, a sheet of mica or the locksmiths use a special tool to open the doors
If you go out you should ALWAYS double lock the door for security so all of the bolts engage
Having been a keyholder in the past agian this could be an good idea as they make a smaller charge just to hold a key for emergencies or if access is required if you are not around
Not keen on key safes on open balconies, too easy to spot and open