Hi Tech is spot on, imho.
That fairing is not structural, and the boeing CDL may even allow ferry flights with part of it off, not sure, can't remember.
Oxygen bottles or even fire agent tanks exploding are rare, almost unheard of events in aviation (I've never heard of one in flight.) But not impossible due to the possibility of internal tank corrosion. Usually, if there's a thermal expansion problem they will blow out the green "blowout discs" on the side of the fuselage which is their function. Now, they can go off like a rocket in the hangar if tipped over and the brass works get knocked off.... but that's because they aren't secured to anything and the thrust produced by the escaping gas is accelerating your unrestrained, hole-punching newly commissioned "bottle-rocket." But the ship's on board O2 tanks, however, are secured in on board racks aren't they? So If one decides to go into orbit something else must of dislodged it, or it was not secured properly to start with. Pretty unlikely guys if you ask me.
More likely are possible signs of black corrosion on the green zinc chromate where the rivet heads are (dripping downwards) in the photos. It's a common sight on old airplanes in inaccessible locations. On the other hand, it may be a harmless stain and there's no way to tell without a closer inspection.
What is odd about this, to me, is the possibility we are seeing a ULD steel pallet bottom that has been grossly deformed. Many of these containers are steel on the bottom and fastened aluminum on the sides (or in the case of a pallet, just steel.) They are fairly heavy gage on the bottom and FAR regs require the total weight of each individual one to be stenciled on the side for weight and balance calculations. It's surprising to me that an explosive decompression alone could deform such a tough structure if that's what I'm seeing, but clearly it sucked it through the [aft fire curtain] of the [aft lower] cargo compartment to the location it is in now; complete with what you would expect to see, shipping debris, on top of a pallet.
Corrosion or improper repair are the usual suspects in my mind (Ask JAL in tokyo about that.) How many times have you seen an ice bucket quietly leaking into the floor back in galley cart land? How many times have you seen overventing stains behind the lav drain masts?
Note to self: Quit eating those free blue icicles on the lawn every morning at my house under the SID flightpath!
