It looks suspiciously like a young bindweed sprout, to me. Does it have a pair of seed leaves atthe base, or does it seem to come from deeper in the earth?

It could be, but to me the leaves are not 'succulent' enough to be Bindweed.
They are certainly growing where there was a lot of the stuff .
Definitely a seedling not a new growth by the way.

Hmm...maybe a solanum cousin, then? I have one in mind, but I can't drag the name out of my brain. Big blue flowers, papery covering over the seed pod.... :/

We have had Solanum of some type in the place. Cannot think of a name either. Quite a common weed too. Not Deadly Nightshade though. BUT I think I would recognise the leaves since we have had it in our gardens before.

Been identified as Alliaria petiolaris, Jack by the hedge Garlic mustard. Should have known, but can only remember seeing it as a fully grown plant before.

Were the cotyledons really long and thin in comparison, it doesn't look like the Garlic mustard I have germinating by the bucketful in my garden. but could be the angle of the photo. does it smell of garlic?

Wish I could post an image of Alliaria in an answer. They match the image perfectly.
https://www.google.com/search?q=alliaria+petiolaris+seedling&client=firefox-b-d&sxsrf=ALeKk01tpst5V7G_iPvNuZXOJsBqnW2MfQ:1617737198157&tbm=isch&source=iu&ictx=1&fir=x-EkbtKHnmiuYM%252C3L9p7A90qRkSPM%252C_&vet=1&usg=AI4_-kSPBzj7Wniqb2Q_8obyN_9C1GrQmg&sa=X&ved=2ahUKEwj-rqTYrOrvAhVTgf0HHeaSDL4Q9QF6BAgbEAE#imgrc=x-EkbtKHnmiuYM

Aren't garlic mustard leaves a more blue green than those? Seem to remember they are a bit rough rather than smooth and shiny but its a long time since i saw any. Also they have serrated edges and the habit is more like nettle.

I just had a look on line and found two distinct pics, one did have similar shaped leaves to these but most were the nettle shape that one would expect. If you let it grow please let us know what it turns out to be. I have been through two flower books page by page and not matched it.

So have I. Trouble with finding id's is that you often need to know the answer before you go looking.

i have some that match this so yes it is garlic mustard, but others with very crinkly leaves.