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I spotted this a bit earlier whilst out walking the dog in a field with a line of HV pylons across it. What's it signify?
It looks quite similar to the ones you see marking the location of publicly accessible defibrillators and AED devices, but not quite the same - and there isn't one of those for a mile or more, so can't be exactly that.

It is, it's just one with a bit more oomph to it
Heart check point for para gliders?
Where they hang it when teams are working on the pylon
There are two sides to a pylon that can be switched out individually. There is a system of flags and wrist bands that ensure people don't climb the wrong side when set to work. You only climb past the signs you have a matching wristband for.
It could be that, or something else. All the markers I'm used to seeing were geometric shapes rather than hearts but a permit to work could easily say only work or apparatus identified with a green heart on a yellow circle. That would make it clear. Especially as you can't work operationally in the electrical industry if you're colourblind.
The avocados on this particular tree aren't ready to be picked yet...they are still healthily growing. The signs with the brown circle instead of the green heart signify that they are ready to be plucked (or plugged as the workers call it).
Is it near an elephant sanctuary?
Earthing point stud for the biggest defib in the country.
Climb up the side of that on those pegs? Not on your ****ing nelly!
You only climb past the signs you have a matching wristband for.
That does leave the question of who is allowed to climb up to put the signs there in the first place?
I think it’s an easy to recognise symbol showing which circuit or loop that pylon’s on. I could be wrong though.
Thank you @supernova - one of the only two actually helpful suggestions so far. Next time I'm in that field, I'll try & check out the next pylon up or down the line to see if it's got one the same.
Obviously pretty much all the other posts are entertaining in their own ways, much as you'd expect of course.