Scantronic 9600 Exit doors ?

Hi
We bought a house with a Scantronic 9600 alarm already fitted and we had the instructions to change the codes for it ,which we did
However there are contacts on most doors in the house (I don't know why)
Anyway the front and rear doors both have contacts on them and I was wondering if we could enter or leave by either of these doors
The reason I ask is because we have only ever set the alarm and exited throu the front door
The other day we went out the rear door and everything seeed ok ,the beeps went off when we exited (after a little while)and the beeps started again when we came in and stopped when we reset the system
However today we set the alarm the beeps started and we went out again locked the door goy in the car and went out
When we returned thou the alarm was making a droning sound(the sound you hear from outside immediately after the beeps stop ) which then goes silent
Today that sound could be heard when we stood outside the door
On enetering the property only the drone type noise was there ..there no beeps and I reset the alarm
The only thing that was showing on the panel was rear door fault or open (can't remember which )
Can you give me any advice as to wether we should be able to use either door for exit and entry
Thanks in advance