I had just fed it an hour or so ago, making it thicker in the process by adding less water than usual, and when I came back to it just now to toss it, there was a distinct sour tinge. I tasted a little of it, and it was definitely sour. Still not much activity, but I'm going to let it go for another day and see if anything comes of it. In the meantime, I'm going to make a poolish either this afternoon or tonight, I think.

Looks like your starter was liquidy like pancake batter in the first post.