Thanks everyone for your suggestions. That one visible from the M74 seems to catch a lot of people's eye - though I've never noticed it myself! However, it's definitely not the one I recall being discussed previously (perhaps not actually on UKC, though).
My own researches have turned up a place in upper Moffat Dale called "Emblem Brae", which is a suggestive name. There does seem to be a rather strangely-shaped forestry plantation on the SE side of the valley which, if you screw your eyes up a bit, you could almost convince yourself might once have been a crest or coat of arms of some kind. It's at grid reference NT 17722 13935 - although unfortunately the OS aerial photograph has it in deep shadow. It's also visible on Google Maps:
http://tinyurl.com/pwdwoc4
I've not yet managed to find out anything useful about the name or the history of Emblem Brae. Am I deluding myself that this could be what I remember reading about?