Why does Facebook chat show me as on my phone when I'm not?

[Bob]

I have always wondered why for some people, chat always displays them as "on mobile" so I had had a friend test this for me and surely enough, apparently chat always displays me as "on mobile" even when my phone is turned off and not in use. Why is this? I have adjusted my notification settings and everything but it still shows me as online.

[Tyler]

If your phone is synchronized with facebook, it will show up as you having a mobile device "online"

[Amber]

Probobly because you were originally logged on you're Facebook with your phone and never log out

[Yahoo mania]

When you download the Facebook Messenger and login to facebook . It will always show you as online even after logging off . Uninstall the facebook app if you're unhappy with this
Another reason could be that you've synchronised your phone with facebook