Is there a way for someone to chat that can鈥檛 be seen when logged into their account from another computer?

I have the password for a person's facebook account (I'm not going to explain why). If I am logged into a computer on a completely different network at the same time they are logged in I can normally see their chats as they occur (no I'm not interested in the details of these chats). I can of course also see and view saved messages. What I want to know is, can they possibly chat to another person, who by chance is on a Samsung mobile, and I cannot see these chats. I can't see how that could be, yet I've got myself convinced that this is what's happening. Any useful and relevant advice would be greatly appreciated. I cannot put a keylogger on their computer.Is there a way for someone to chat that can鈥檛 be seen when logged into their account from another computer?
Yes

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