Ahh the usual electrician issue ( some of us are here too, but come from a security background).
I suggest you ask him to read here if he is finding it difficult dealing with "millivolts" .
Then tell him to go forth if he cannot understand he has made an error and it is his time not yours to pay for, as in NOT YOU.
And of course you were out and did not witness anything.
Try reading the logs, print them here. Alarm panels do not lie. People do!!!
Alarm
October 2012