Knog Bags. I have one of these and it fits the description. I works just fine as a messenger bag, it has a padded laptop pocket, and it also has an easily attached and removed device to turn it into a pannier. It even has a pull over rain jacket to keep it dry. With the bicycle attachment off nobody knows it is also a pannier. I've been using it much more as a bag than a pannier, and I work in a very corporate type office and don't feel in the least out-of-place with it. (Mine is black, the color that comes up on the link is fugly.)
I looked at the Ortlieb bags, the Arkel, and a few others, and they all looked too much like panniers. This one doesn't.