There is a total lack of transparency about just about every aspect of the collecting and - especially - trade in antiquities. For some reason the world has got used to accepting that this "has to be" some kind of a norm, without really asking why and whether it is beneficial to treat a common heritage like the archaeological resource without full transparency. Obviously it is not, it hides a whole load of egregious examples of extremely bad practice from any kind of public scrutiny.
"Metal detecting" is notorious in this regard. Artefact hunters shield all sorts of information from public scrutiny on the spurious grounds that the "only reason is" that if they do not, unscrupulous individuals in the metal detector owning community might use this information to "poach" the artefacts from "their" most "productive" sites. Of course it is precisely such sites that heritage professionals need to know about at the earliest opportunity to protect them from being exploited erosively by artefact hunting. It does not take much of an imagination to realise that this too is a reason why artefact hunters (interested in getting stuff out of these sites for their own private use) are not too keen on providing this information.
Metal detectorists like to persuade the public that the "majority of them are responsible" and it is only a "small minority of black sheep" that are the cause of the problems with artefact hunting. That is belied by what goes on in some clubs and the contents of many of the internet discussion groups in which they gather and discuss candidly what they do or want to do - most often received wholly unjudgementally by the rest of the "responsible" membership. It is for this reason that public scrutiny of these metal detecting forums would soon betray the extent of the "portable antiquity scam". It is for this reason that the majority of artefact hunters' forums are not open access, but access to the posts is restricted to approved members only. The forums take great care to make sure that critical eyes do not get to see what goes on in the closed sections of these forums.
Even among themselves, "metal detectorists" seldom write on their forums under their own name, but under an assumed "screen name" to hide their real identity. PAS does not report finders' names, so no member of the public without special access to its database can check precisely how many items a self-proclaimed "responsible metal detectorist" has actually reported. The exact place artefacts have been taken from is never reported, so again it is impossible to say whether a reported find has come actually from a known site or merely an adjacent area.
All these things conspire to hide the actual patterns nature and scale of artefact collecting activity going on under the umbrella of the Portable Antiquities Scheme.