The one about not bothering those people who are using the beach for sunbathing, swimming, playing etc... - I generally hunt either early in the morning or late at night so I tend to forget this one since it rarely comes up for me.
This one that you posted is a good one also, but it has such a "gray area" associated with it, and perhaps it's the one that's at issue here.
3. Dont intercept the path of another detectorist hoping to snag something before they do
Personally, when I'm detecting, I am focusing at least 90% on watching my detector head and listening for the tones. If I've ever been in a situation where I've "cut off" another detectorist, I've never done it intentionally.
I can say that there have been a few times where I've looked up and seen another detectorist ahead of me by 30-50 yards or so seemingly running along the same path I am - in those situations, unless there's a specific reason I'm looking along this stretch (look of the beach, searching for something specific I know is there, or having a hot streak), I'll generally start looking in another area.
Other than the crazy detectorist I met one time who decided he was going to shadow me and detect all around me and annoy me, I've never really had any issues - at least not enough that I would go up to someone and confront them - it would have to be extremely blatant and probably happen more than once for me to even notice.
In general, the beach is a darn big area and there's room for everyone
That said, I will start taking more note of where I am in relation to other detectorists when out hunting - I can certainly see where one might think their "area" is being encroached upon.
I guess I'm just a Pollyanna most of the time - we should all be able to get along
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