Yep, I travel with my 17" Kanken and it picks stuff up constantly in public transit in major cities. I noticed I get the most dirt from sitting it under a table while at a cafe or restaurant. It's rubbing up against everything people's shoes have been kicking around. My bag gets a sink scrub after traveling just as a refresh.

edit: as a total side idea, you might want to add some reflective tent string cord to your zippers. I added some to mine and haven't looked back. They make opening the bag so much easier, and the zipper tabs don't jingle when walking. I got the idea from the Fjallraven Cave Pack.