If it's an alcohol fueled dummy weapon, it will cycle and spit out the shells by design. It will make a pop but they lack that concussive pressure wave if you are nearby or in an enclosed building.
In your attached pic, you can see the slide isn't fully forward. Now seeing the bullet shell, I would make the logical assumption that the slide is returning forward.
Prior to you pointing out that shell, I presumed it was a split second prior to the bullet exiting the barrel.