Cant believe we have 3 pages about a lost panel..
The reason why this panel separates is that the support bracket design isn't rigid enough to stop the panel from vibrating and that leads to the screws migrating out. Its not unusual to see screws on a turnaround that have started to migrate out and this can be exacerbated by having worn anchor nuts or incorrect length screws - that being said I have seen this panel lost on a newish aircraft from Boeing that hadn't been through base maintenance and had never had that panel removed during airline service.