Thanks all for the sympathy.
@Gregoryniss, well yes, there's that, except that I didn't really have any GAS before this happened. I was really perfectly happy w/ my setup and wanted to keep it forever.
@swoop, It was a messenger style bag with just one shoulder strap. Normally, I wear a backpack which of course is much smarter (2 straps) but today I just needed a few things and they all fit in this one bag I have perfectly. It's made by Acronym, a german botique technical apparel brand ... they're really quite something. After retrieving the bag it looks like the shoulder strap where it's attached to the bag on one end just came undone from the loop style buckle. It's a hard lesson for sure, but from now on if I need to carry any gear with me while riding I'll be doing it in my backpack and I'll be making sure the ends of the straps are sewed off so there's no way they can slip through the buckles.
Right after the shoulder strap came off, the pack was actually just sitting on my back seat for a little while kind of caught in the wind tunnel my body creates. I didn't grab for it because I figured the strap had just gotten looser, not disconnected, so I figured at some point it would just fall off the seat and my shoulder would catch it again and all would be fine ... but then 30 seconds later I could tell there was nothing back there any longer. It all happened at a pretty congested part where traffic was slowing from 75 to 20 pretty quickly, so I'm glad I kept my head on straight and payed attention to my driving rather than worrying about my stuff ... it could have been much worse. But still, after it's all over ... it was a sad day.
@all ... if any of you have a good lead on a used black paint MP, I'd appreciate the link. Doesn't matter how worn in it is.