Hey addicted2alice - Clear your cache in Chrome (home computer) and I dare say you'll see the issue as it was at work for you:) Can I possibly ask what theme you're using?
And yes - to confirm splashingpixels - all fine with a vanilla set up and twenty twelve my end as well - it only shows up on twenty twelve when tt has been activated after the original theme was installed and then replaced with twenty twelve - thanks so much for mentioning this - so definitely something to do with the my theme even though the developers say otherwise.
No time to sort for next few days - a bug I can live with for a bit - but will come back if I find a fix addicted2alice:)