Is there an app tolocate your device even when it isnt online?

No, "Find my iPhone" is the only app that you can use to locate the device, which has to be online or connected to a mobile data network to be seen.

if it's not online to the Internet which is use to tell you the location then by what means would it inform you of the location ?

without being online it would not even realise that you wanted to get a fix