What do the facebook chat status icons mean? (2014)?

[lil will]

When looking at the chat menu on facebook, there are various symbols to the right of my friend's names. There is the one with the phone that says how long since they were last online, but then there is one with the phone icon, but nothing next to it. What does that mean?

Here is a picture I made. The symbol I don't understand is the third one down:

http://imgur.com/zvxujow


I am just curious if anyone knows exactly what it means.
I know it doesn't mean online using mobile, because obviously that is what the second icon means...

[Chris]

1 Mobile online
2 computer online
3 not online

[Angeleyes]

The third one down is when they are using their mobile to go on facebook.
1. Online using Computer, 2. Online using facebook. 3. Accessing Facebook through mobile. 4. Time since they were last active on facebook via mobile.