The "Nighthawks" are commonly portrayed as "the only problem" which metal detector use on archaeological sites produces. Nobody really knows how many individuals go out and search for artefacts illegally. Neither is it known to what degree individuals go out artefact hunting most of the time in accordance with the law, and once or twice for one reason or another illegally, or vice-versa. Most of the time those doing illegal activities do not tend to boast about it in polite company. A few years back English Heritage produced a report on so-called "nighthawking" (which is a misnomer, many unscrupulous detector users probably search sites illegally in broad daylight if the hedges are high enough and the site cannot be seen from the road or farmhouse). It concluded that the scale of the activity was down compared to previous years, though these conclusions remain tentative and uncheckable by the means applied at the time the report was compiled. It was suggested in some way the Portable Antiquities Scheme had played a part turning black devils into white angels. Personally I very much doubt it, and refer the reader to a series of posts on my main blog for the reasons why.