Enderkay, have you installed Seasons? That looks like a spot intended for rain puddles (they are like almost-invisible oval mats). It seems to be remarkably persistent: even though I've disabled Seasons in my 4x4 Dynasty, one of the mystery mats appeared in the second house's games room and I had to avoid that spot in Build/Buy modes.
Like Trip said, I'm pretty sure those are the SocialJig mats. Puddles just stay as puddles, I think. (I always have rain turned off though, so I'm not sure.)
@enderkay
Go ahead and try using testingcheats to delete that object.
Oops, I should have mentioned this before. The problem is, it's not an object. There is literally nothing there. No SocialJig mat, no nothing. Just...space. Impenetrable space. I've never had something like it in my game so I have no idea what's happening. That's why I wondered if it might be the IF, since it's right where the thing used to sit at night.
I've worked around it for now by building my house slightly to the side. There's just enough room for a decent house, which is good. I'll just have to put bushes around the spot or something. (Is MoveObjects considered a building cheat in this case? That way I can stick a bush on top of the space and not worry about it anymore.)
Since it's not really pressing anymore, I guess I can leave it. I'll just play around with a copy of the file and see what I can do to make it go away, just in case it happens again.