The problem is that it detects the intruder only at the time in which analysis is performed, whereby if the "thief" WiFi is not connected now can not find out until we agree.
However, to know if they are connected at any given time, access the router. To this we must introduce generally 192.168.0.1 or 192.168.1.1. in the browser or go to Finder Windows (Start / search engine) and type cmd.exe. a black screen on which we write "ipconfig / all" and, once this operation will appear, "Wireless LAN Connection" and then "Physical Address."
Thus, the user can know the encryption system. It is very common that provide internet services opt for the WEP system, considered the most insecure and less desirable.