Are these the leaves of a greenbrier vine, or something else entirely?

It looks as if it could be a Smilax (greenbriar) species. Are the stems thorny?

It doesn't have any visible thorns, hence my curiosity.

I think I see a thorn or two. Can you check it again?

On the surrounding stalks there are thorns (the area has recently been cut-back), but not on the young plant. I'm assuming that this is an offshoot of a pruned stalk of greenbrier? I followed the shoot into the soil with my hand, and noticed no connection to a mother/main root. I don't doubt that it is greenbrier and that you're correct.. I've just never noticed such a pretty greenbrier leaf

I'm not sure it's Greenbriar, but some species do have variegated leaves.

Compare to this one:
Saw Greenbrier (Smilax bona-nox)