Fixpix, that was my first idea, but I don't know the group that well. Interestingly, I know of no plants within a couple blocks, so I don't know how the seeds got to my yard. There are several seedlings, so there must have been a significant source. If birds eat the seeds that would be one option, but I didn't think they did..
I suppose I should compare against some of my potted plants, such as Calliandra californica, which I know seeded last year and is a few yards away. I don't think the leaves are quite right, but I'll take a closer look.
I used to have a Caesalpinia gilliesii plant, so it's a possibility based on that, but I don't think it ever fruited. The leaves don't look quite right to me, though. I don't have a plant handy for a direct comparison. I also have Caesalpinia pulcherrima which looks quite a bit different. It's a prodigious bloomer and fruiter, so I have plenty of C. pulcherrima seedlings to compare.
EDIT: Here is a picture of my C. gilliesii plant.
There are albizia plants in the neighborhood, too. I'll have to check where the closest one is. I think it's at least two blocks away, but there may be a closer one I didn't notice. If it is an albizia, when is the earliest I can expect a flower? I don't know if I've ever seen a young one in bloom.
Do the leaves close up at night? Albizia leaves do. Although I'm not sure if seedlings would have that behavior yet or not. Also many other things in that family have leaves that close at night too, so if it does close up it doesn't conclusively prove that it is Albizia but it would at least keep it on the list of possibilities.
Yes, they close up at night. Surveying all my other plants, of all the pinnate plants, about half of them close up at night. This includes Caesalpinia pulcherrima. I thought I'd mention that since a caesalpinia was suggested as a possible ID for this plant.