9200 series Alarm?

The alarm as worked no problems for over the five years we have been in house, and had it serviced on moving in.
Last night we were woke up with the outside bell ringing, (alarm not set), put in code, and did nothing to stop bell, went off after about 10min, Again went off 2 hours later, opened control box removed few connections, and bell stopped, and did not ring again.
Can anyone help why alarm bell outside went off without alarm being set, and how do I fix.

Ray
November 2012