If you are sure nobody sees you, use cones. When I see somebody roaming somewhere in field, especially far far away from village I always make a point in my garmin. I'm curious why and what he is doing there 
. I will return and check that place when he is away 
But if I saw cones or equal marks, be sure I would be there next day! 
Navigate taking into consideration showel trace/foot steps on flat ploughed ground. Use noticeable stones instead cones.
PS.
I'm a legal day/night hawker
Hmmm, a "legal day/night hawker", what does that mean exactly? Suppose you
did see me marking out the land, how keen are you going to be to be to get "there next day"? Suppose you didn't know who owned the land, would you go for a little peek anyway?
I hope you are not the kind of person who makes it necessary for me to have to dress up like an SAS reservist in full DPM when I'm out detecting, the kind that won't butt out and mind his own business, who would rather ride on the coat-tails of my success?*
I know for a fact that one farmer round here agreed to let one chap on his land but was suddenly deluged with other people demanding permission too.
Firstly, he assumed that there was something on his property that he was unaware of, and he assumed the first chap had found something and word had got out, but that he was being cut out of the loop. Then, when he refused anyone else, he got the response "Why not? You let him on there..."
Apart from not needing to be harassed on his own doorstep, he found that bits of his land were being chopped up by either people he'd refused permission, but who had decided to have a little wander anyway, or people who were so keen to find out what the legit chap was up to that they neglected to ask permission. The inevitable result was that the farmer now lets no-one on his land, full stop. It hasn't stopped people going for a naughty dig, and he absolutely blames the poor legit detectorist because all the problems kicked off when he gave him permission, after years of his father refusing any requests. You don't need to be Sherlock Holmes to work out that people had seen someone in a field with a detector and decided they wanted some of the action.
Now, I'm sure that farmer talks to his neighbours, he certainly talked in the pub to my brother who recounted the tale to me...
I'll be charitable and say your comment was tongue-in-cheek, but it's sad to think there certainly will be people who will have no hesitation in steaming in where others are enjoying their hobby legitimately, perhaps after having spent a long time researching the site, building up a relationship with the land owner and sweetening them up to build trust and get that all-important permission.
Strolling over for a chat if you see me in a field is one thing, natural curiosity as to what I'm up to, but don't ask whose land it is (sod off,do your own homework!), or what I'm hoping to find (anything that isn't a pop tin or horseshoe, really). Better you stick to your own permissions, there's plenty of untapped land out there, despite what people think.
I know how receptive I'd be to anyone who spotted me gridding a field then turned up with their machine, either having shot round to badger the farmer for permission, or assuming they could join me, that the land was now "open". They'd certainly get something Anglo-Saxon - an earful of expletives for a start!
* Meh, yeah OK, if I spied someone laying out a grid I'd be over like a shot to find out what was happening, but not with my machine, nor with the intention of getting in there. If someone's beaten me to it with a farmer, my tough luck until they give up!
