I sent somebody a message on Facebook, and their active timestamp now reads "active 1 hour ago", but the message says that it has been seen, even though I sent it less than an hour ago. It didn't say 'seen' as soon as I sent it, but rather about 20 minutes later, but according to the active time stamp the person was not active when the message was seen.
The reading on the browser and on the messenger app agree with each other in both respects.
How does this happen?
There are certain ways you can appear to be inactive on Facebook even when you're logged in.
If someone has their online status hidden, it will look like they are offline, but this is a separate condition than whether or not a message has been read. So someone who doesn't appear to be online could still read a message, which would result in the message showing as "read" even when they don't appear to be logged on.
I suspect it's because of the feature that opens the messages in a small Facebook tab. If there's an open window of Facebook, it will probably be opened that way.
Now, the active 1 hour ago part, might be identified by the idleness on the page - can be that the user left Facebook on the active window/tab on his browser and went away or when Facebook is still open in the background (another tab / unfocused tab/window).
To disable the Facebook popup tabs, you can click the Settings gear ⚙ of the messenger (inside Facebook) and choose to disable it.
