I am in the same boat. Go to "Tax Tools" > "Print Center" and click that you want to print not just your return, but also all Federal and TurboTax worksheets. Then use Ctrl-F and search for the number to see where it appears in the worksheets. In my case, it's coming from line 18d, "positive adjustments" on the tax payments worksheet. I don't have any idea what that means, but at least it's a start. (As far as I can tell, the number is completely made up. I had a terrible experience with this buggy software this year, and I will be going H&R Block next year, I think.)
