It could be peppervine; does it get bunches of little white flowers later in the year? It could also be a grape. Either of them can be deposited by birds.

Your location would be helpful - state and zone.

I am in Zone 7 - New Jersey. I usually pull them before they get to big - so I dont know if they get flowers later in the year. They grow mega fast.

If you're in NJ, as I am, it's peppervine. I've gotten pretty good at recognizing the very young plants, and I get most of them then. The birds love the berries, and the seedlings tend to be near wherever the birds sit and poop - by the fence, under a well-branched tree, and so on.

Here's in Invasive Plant Atlas page on this dreadful plant; there is a picture of the seedlings towards the bottom of the page:https://www.invasiveplantatlas.org/subject.html?sub=3007