I?ve always organised a rendezvous point where the project managers who authorize the hot works, and/or the leading hand on the hot works team, go to, to meet with the responding emergency services.
If the hot works team is large enough, working in teams of two, they are required to sweep the area that they have been working in to conform that they are, or aren?t, the cause of the alarm, before also reporting to the RV, or radioing their findings into their leading hand/project manager before going to the assembly point or dispersing.
It can take some time to get contractors to comply with the procedure, but most are pretty good and understand the implications of not complying. Which, by the way, included forfeiture of a percentage of their fee if they didn?t comply and/or where the cause of the alarm.