I ran into the same thing on a cooking blog (which I had visited many times before) a few weeks back. I was pretty mystified but like you, spidey sense kept me from running the code. It's insidious because it looks like it's coming from Captcha. I do have uBO Lite running on Chrome. Tried the website in Firefox and also got the same manual run thing.

I wasn't sure what to do about it. Obviously I was unable to notify the blog owner. I came back to the website about a week later and it didn't come up again.