It is not a plant I have ever seen personally but take a look at Phyllanthus urinaria aka Chamberbitter aka Mimosa Weed and see if it looks like your weed.

Looks a bit big for that, but it's hard to gauge the scale in this pic. Gardennewbie, you can add more pics by replying to this, one pic per reply.

There's no scale in the picture - can you tell us how big it is, or put something in the picture to show scale. I was assuming those were grass blades behind and that therefore the plant was just a couple of inches tall. Is that too big for Chamberbitter?

I think it probably is chamberbitter.