Hi Steve, whatever type of sensor on zone 5 is indicating that it is “open”. At the end of the exit time, as it is still open the exit tone continues to alert you to this fact. You need to find out what zone 5 is, and fix that. Just for your info the system although continuing to make the exit noise will in fact have set, automatically omitting zone 5.
If other pirs on the system are ok, ie have power at them, i suggest it may be a door contact which has failed. Best to find out what zone 5 actually is before going any further.