Fault on Tunstall house alarm?

I have a TS 500 Tunstall house alarm panel. I set the panel to part guard when going to bed ( omitting zone 3 when going to bed). 2 nights ago the alarm went off during the night. Could not find anything wrong but zone 4 was on constant + zone 3 was flashing ( zone 3 had been omitted during the night, but I have to pass it to switch alarm off). I linked out the normally closed circuit on zone 4. Last night alarm went off again during the night, this time zone 5 was on constant + zone 3 was flashing. Any ideas. All zones mentioned or on sensors. Panel will also reset ok. Had panel on full guard yesterday while out during the day and had no false alarm