I have notices that quite a few detectorists new to the hobby are posting images of their fields etc in places like facebook as a bit of a show off of crop marks etc. I don't think it would take much for night hawkers to find the locations (based on the posters location) as it seems most new permissions are near to where you live. Buy all means share the find pictures but be very careful of showing the locations. So if you are new to the hobby be a bit discrete of what you show. As for me if anyone asks me what I have found, unless its the land owner I always say "na all rubbish so far" :-)