Confused: Facebook Chat Settings, help?

[Summer]

My questions are regarding Facebook chat. I have looked around and run my own experiements but what have others learned? Please answer clear and succinctly as possible. Backstory below.

* I use to see someone as active (green dot) on chat, but now they are never on my sidebar.
* Now when I type their name I see a mobile icon next to their name with no time/date.
Which makes me think they are active on their mobile device.
* Then I turned chat off and saw that persons pic appears, so...

1. What does it mean in FB chat when a friend has a mobile icon, but no time/date next to it?

2. What does it mean when you turn chat off and the sidebar goes grey and it shows that friends pic?> Meaning a friend off yours you did not see as active via green dot or mobile but when the sidebar goes grey from turning off chat their pic appears?

[Amanda]

1. That person logged in Facebook using mobile, they turn off the chat.