In some of my overhead cabinets there are four inch round discs covered in fabric. The discs are held in place by buttons. Do the buttons unscrew or do I pry them off?
The buttons can be pried off with a flat screwdriver or similar. A screwhead is under the button to hold the disk in place. Under the disk you will probably find an attachment for an airhorn, antenna, etc.
Or if it is in your slide access to the slide pin receiver.