I found and checked my poisonous berry list - no help. Doesn't mean it isn't, of course. It also could be an imported plant that has escaped from cultivation, then it could be anything.
This thread points out the very real need for a plant book, organized by berry colour! The list I have I did years ago, and it was only for poisonous plants. Might be a project I'll take on, not right now, though, too many other things on the go.
I wonder if there's a website that lists berries by colour.........
Eric, if you still find the plant, mark it with something, including your GPS so you can find it next year when it's in flower, assuming it's not an annual. Also if you can record as much info as possible, a local university could help.
Things like the aforementioned margin, and leaf stalk, whether leaves are coarse or not, hairy on top, bottom etc., hollow stem?, stem round or square, number of seeds in berries, size of berries, leaves, etc. well, you know what's needed.
Doc