And finally, after many, many hours of sniffing, I discovered the source of the BO.
And here it is:

Yes, that's right. It's the Trident Tropical Twist pack of gum I was carrying in my backpack. And trust me, if this doesn't smell like BO I don't know what does! Seriously. This shit stinks. Stupid crappy BO gum.
At least now I know why the security guards turned me down when I volunteered for the full body search.