Any ideas apart form the College of Policing, National Police Chief’s Council and Opengrey, please. I want to search articles for my literature review.
Global Policing Database and Public Safety Canada are two good sources. But grey lit is really hard to find. Basically, you have to know the subject very well, so that you are aware of the whos and wheres. I suggest you including an extra step to your search procedure: applying the inclusion criteria to the references of the related papers – e.g. those elegible for full-text analysis. Papers written by veterans/experts will surely reference good grey lit.
Are you aware of The Centre for the Study of Missing Persons (CSMP) hub at Portsmouth? they would be the best people to ask...?
Are you aware of the library that the College of Policing has - their librarians are very helpful, and they have lots of grey lit in there, if you can get access.
The NCA have a UK Missing Persons Bureau and do (or did) produce an annual Missing Persons Data Report. They too may advise.
Finally, the Police foundation prob have one or two reports, and are one of the few authors with no point of view to defend.