Generally there will be no problems with the squib circuit in civil helicopters provided the system receives its required maintenance. Analysis of the few that I am aware of revealed system defects in each case (except for one where there was a pilot defect) that should have been detected with correct maintenance. None were as result of an external "trigger".
The most common problems we have observed with un-commanded firing (and thank God there are not many) is simply the result of a lack of necessary maintenance, a lack of a good bonding throughout the system and/or very occasionally, poor electrical system design. The problems magnify where the hoist is classified as role equipment and is being installed and removed by the flight crew. (Don't get me wrong any of you flight crew that do carry out the on/off tasks. Some of you are possibly properly trained in the use of the correct test equipment to ensure the hoist system is correctly earthed/bonded after installation)
For the record it was simply poor earthing/bonding that caused the Sydney accident. Over a period of time deterioration of the earthing/bonding went un-noticed and eventually the system finally found a ground through the squib circuit. Note that that particular installation was a local design installation, not a factory setup.