My two daughters live in a complex where a pair of goons are hitting apartments. The routine is for the perps to walk up to the back door (inside a fenced area) knock on the door and if there is no response, they kick the door in and take what they want (typically jewelry). Since it is impossible to hear the door being knocked on from upstairs, there is a high probability the perps will walk in on an occupied apartment. The sticker is one preventive measure however ineffective.
BTW, one daughter is armed and very motivated. The other is growing in interest.