How to keep someone from seeing I read their Facebook message?

[roundthread003]

If you intend to read people's messages with their knowledge, then I suggest using a normal Internet Browser on your computer as you can mark their messages 'Mark as unread'.

Another important hint, NEVER close a chat window that is open... as it'll stay closed when you next login, so it'll become obvious that somebody has been on their account.

I'd forget about using the app on your phone as you have limited control navigating with Facebook, instead use your computer and simply mark the messages as unread after you've read them. Also consider keeping a copied log of the messages in a text file so you can read them later.