You will know what devices are in that zone, so you could hazard a best guess as to the cable run. One would start by moving the end of line resister back to the panel, this would prove the end of line device and the panel firstly. Then move the end of line to what you believe would be half way along the zone, this would then give you an idea of which half the fault is.
Might be worth checking the connections on detector bases. You will find if a system has been in for a long time and detectors are close to doors etc, the continual banging of a door shut can loosing connections in detector bases.
Check that all the detectors on that zone are correctly fitted into their base. We've found on numerous occassions people slightly turn them so they look like they are still in the base but this has broken the continuity connection therefore bringing up a fault on the panel.
Do you have any external manual call points that could be subject to water damage?
Did the fault occur after a particular instance?
Do you have a current maintenance contract that you could call the company out if needed?
