Sourdough - I've been out this morning and learnt a few things which may help. (The helpful bits about the Z are at the bottom. The rest is only helpful if you have a wife )
I hit this spot nice and early before it got too hot. Detected from about 0530-1000 hrs.
GPZ was running great and only about 40 metres from the EPT lines - high yield, normal, sens 2. Anything higher on the sens and it started to waver a fair bit. Kept switching between normal and difficult as there were a fair few hot rocks and a bit of red clay - but mostly normal.
Dug quite a bit of crap early.
Dug one very deep hole, right next to a tree, just next to the line of a long cut out section the old timers had dug and got down to some very hard sandstone, clay, rocky mixture. Very confident at this point I was onto something decent. Approx 2 feet deep and onto bedrock - nice!! Couldn't dig any more at this point with my little pick and was positive I have cleaned the hole out best as I could.
So I called the wife . It was 0700 hrs. I woke her up. Please bring crow bar, axe (tree root in the way) and the SDC to try and pinpoint a bit. Last time I did this it did not end well - a big steel bolt at about 1.2 metres!!
Wife and 2 of the kids arrive. Eldest son gets rid of the root. Turn on SDC. Target in hole. Getting rid of the root put some dirt back in the hole. Scrape, scrape with the little pick. SDC in - target now out . Wife sits on chair (who brings a chair??)
A lovely piece of flat steel about the size of a matchbox and maybe 4mm thick. Must have been stuck near the end of the root and that's why I couldn't pinpoint it better. Son must have dislodged it.
Unrepeatable words from wife...
Moral of the story - take all gear that may possibly be required and let sleeping dogs lie.
BUT, the thing I learnt about the Z. Before I really dug much out of the big hole I switched from normal to difficult to see if maybe it was just ground noise or a root - the signal was only weak at the start. Still made noise in difficult so was confident of a target. Then tried to go to general and extra deep ground modes with the thinking that if extra deep could still hear it then it would definitely be a target and it was also going to require a fair hole. But they simply would not work! HY/normal, HY/difficult worked a dream - any combination that included general or extra deep ground modes went crazy with EMI even with sens of 1. Why this is I have no idea but to be honest I minimal understanding of how detectors even work so I'll leave that for others to surmise.
Sourdough, not sure what modes you hunt in but maybe when you've tried near the power lines you may have been in general or e/deep?
Also, I think my theory is right about that spot not having been detected before considering the huge amount of small steel, lead shot, wire, nails and tin that I found in about 4 hrs all within a 10 X 15 metre area. I'm assuming due to the power lines being there. But no yellow...yet.