Messenger Icons

I sent someone who isn't a friend a message on Facebook. I know it would have gone to them as a message request so I kept checking to see if they had read it on messenger. The logo which they had was the white and blue bolt but after a while it changed to the grey f but the message still only had the sent icon (blue circle and blue tick)

Has the message been read? How come the bolt logo changed to a grey f?


Thanks