A grey area they might be an intruder? there was a story a few years ago off an RAC man got called to car with the keys locked in and when he arrived there was a man next to the car. RAC man got the keys out in just a few mins and the man drove off, Then the owner of the car arrived to find the RAC guy and his car was gone.
I might do if i looked like a DIY install with no installer details and it seemed 100% that they had a right to disarm the system. I would help a neighbour like a shot