I’ve looked at this issue on a number of occasions and have concluded that an area sweep by fire wardens is amongst the cheapest and most reliable system to adopt. That is providing that there are fire wardens on duty at the time of the alarm. Many companies now have twenty four hour access for their staff or at very least operate flexitime which means that people can be on the premises at all sorts of times of the day or night and it’s just not possible to ensure that a fire warden is present at all material times.
I looked at a couple of automated systems that worked on PIR foot fall counters, but they are at best 99% accurate, which isn’t too bad if the building has a daily “closed” period during which the counters can be reset but not much use otherwise as the margin of error grows and grows.
One other system that was being worked on while I was looking at this was a computerised counting system that used CCTV to recognise people as they walked in and out of a building and created a running total of occupants. Very clever, but it had a few teething problems as you can imagine.
The big problem with the automated systems is that the detection devices/cameras have to be placed over every access and egress point, and if the building operates on a phased evacuation or PHE regime this can be very expensive.