What does it mean when one has the mobile image with no time duration in Chat Box?

I customized my chat box to be online only for some friends. For online friends of this list I got like "web" or an image of a mobile with a time duration( I think it represents when this friend was online), So what about just a mobile image like the following:

It means that they are online from their phone, ie, in an invisible mode. Just like how you had turned off your chat, they would have done the same thing, from a mobile. It also means that they use Facebook Messenger for their mobile phone. :)

it normally means they haven't been online for more than 2 days. Even if they stopped using the messenger app it will still say they were active so many days ago. After they've been inactive for up to 2 days or more, it doesn't show last active anymore.

This is a kind of bug. You can take a screenshot and report about this to Facebook here:

User has turned off chat but is accessing Facebook or has left Facebook open in an app or web-app.