Metal detecting tracking

Your watch can track where you are right now, I'm not aware of a way to have it alert you if you go into a certain area (in your case, somewhere you have been already).

You could always plan a route in advance and use that with the watches "off course" alert within the walk activity profile. That would tell you if you go away from the planned route. 

What you could do is plan an area you want to detect within using Komoot or Garmin Connect & map the area prior to detecting there. Then upload the planned "routes" to your watch to use in a walking activity (with the watch guiding you where to walk) 

You could divide an area, say a rectangular field, into 4 smaller zones. You then plot routes for each zone, making sure they don't overlap and follow them on your watch

For the first zone, start at the top left of the field and plan the route straight down to the bottom left. Then turn at the bottom and plan the line to the right of that, going back up to the top of the field. Repeat this up and down motion going to the right of the last track each time you reach the bottom of the field. 

Hope that makes sense, and maybe helps? 

Thanks so much for your reply,  fantastic advice