Just after I returned to London, I discovered that the removable accessory bar for my 2008 Topeak Compact Handlebar Bag was missing. Doubtless, it was pinched by a hairy, shifty South-American I saw in the railway station. ;-)
The piece in question is the hollow tube on top the mounting bracket that allows you to attach accessories like bells to your bike despite having the bag mounted. Topeak sells a replacement mounting bracket which unfortunately isn't compatible with the bag I bought in 2008. I am half tempted to get a whole new bag as the new, incompatible mounting bracket is said to be easier to use. The old system led to pinched fingers.