If you look around the edge of the glass you should be able to see a small plastic hoop approx 3-4mm diameter, this IRC is only visible part of the locking ring. It is possible that this has just vibrated loose over a period of time, can't remember which way to turn it to get it to lock but when it does lock it does so with a positive click, go the wrong and you will loosen it completely.
Mine started to work loose more and more often to the point one day when driving down the road I saw it start to wobble, managed to get the window open and caught it.
I put it back in with some padded double sided tape and have had no issues since.
