Bike had one of those stupid Ventura upright racks Aussies love. Not much horizontal space on those things, guess the bag just worked its way off.
And yeah, I don't think zips have any place on serious luggage, although they can't really be avoided on tank bags.
When I travel I keep my document folder tied to a ring on my tank bag so I can pull it out quickly at checkpoints to show whatever the cops/soldiers want to see.
This arrangement came in handy when I did my Iron Butt Saddlesore 1000 a few years ago - I was well into the ride, having done Sneckie to Thurso, Durness then south to Sedbergh on the western side of the Yorkshire Dales and through the Dales to Richmond. Gassed up in Richmond and hit the A1 towards Edinburgh. Fanging along the A1 I nearly shat myself when suddenly I was slapped repeatedly on the shoulder. Bear in mind it's gone 10 and pitch black. Turned out it was my document wallet which had slipped out of its pocket and was flying around on the end of its string behind me. Fortunately I had zipped up the wallet or I could have lost all my receipts, which would have made the whole ride a waste of time. Saved by the string!