How do people see when you're using facebook?

[RJ23]

If you've turned the chat function off, and you're not 'doing' anything eg. posting statuses, playing games, liking photos etc, how can people tell when you're online?

I presumed they couldn't if you weren't on chat but even though I've turned chat off, there's someone who will always message me when I'm online and never when I'm offline. I was wondering how they can tell I'm using facebook. Does it come up on their phone or something? It's just one person who does it.
I'm also just using it on a computer not a smartphone.

[Kierstyn M]

you have to "hide" its dumb but its appearing invisible. Then you sign off. So no one can send you stuff. But they could still send you regular long messages.

[?]

It might be due to you having the messenger app on your mobile phone, its shows up as online if its running in the background?

[Malikah Simmons]

If you have the messenger app on your phone then that's probably what it is. It won't say your online on web but it will say you're available on mobile all the time. So when they send you a message it comes straight to your phone. And it has a little phone icon by your name to let them know you're mobile.