... Think about it a while.. if you start with an assumption that 600 people were murdered, you have a starting estimate and work down. Street CCTV for a number of streets around can would indicate whether more people left the area than arrived . If the police were investigating a single murder they would typically do that anyway. Were it an air traffic incident, that type of rigour would already be in place.
If people were in the building illegally, would that not swell the numbers rather than reduce them?
The flats were leased to known people who were legal, they had children attending schools, crèche and doctors. Residents were in employment or were on welfare. .. These people had mobile phones. At this stage it would have been expected that a list of missing must exist. Under privacy laws the names should be withheld but not the number of missing.
That the police have stated there there is a criminal investigation underway, and one of the outcomes where loss of life is concerned is murder or manslaughter indictments. So my opening statement is not fanciful.
CCTV is inaccurate because of blind spots, areas of no coverage, access & egress routed out of view etc. Add in the factor of counting the same person twice due to it being impossible to positively identify each and every single person moving around, and CCTV is very inaccurate. It's ok for tracking an individual or small group, but no use for this purpose. Mobile telephone analysis is equally vague, usually giving a radius around a mast of anywhere between a few hundred metres and several kilometres. If you are lucky you might get a 120 degree arc and a range, but that's not the norm. It's not as easy as it looks in the telly crime dramas. So, CCTV and mobile phone analysis isn't going to provide an answer.
Human remains within the building, if there are any, will provide the most accurate estimate of fatalities. It's going to be a fingertip type search of every corner and I don't know if the building is safe enough for teams to go in and search to that level of detail. If the structure has been weakened by the heat, it may be in danger of collapsing.
The council will have records of who is registered to live there, so ticking them off the dead or alive list will be a piece of cake. It's the ones who where in there as guests, or living there illegally, that will be hard to count.