Mac OS X cannot find the hard drive (anywhere) I suspect this is a dead hard drive issue:


blinking question mark folder at startup
resetting PRAM has no effect
single user mode not available with Cmd-S


When I get to Internet Recovery mode, the drive does not show up, either in the Startup Disk dialog, in Disk Utility or in Terminal with ls -a /Volumes

Is there anything I forgot to check before turning to Dropbox to get my files and Apple to get a new drive?

(It's all very disappointing, because this is a quite recent MBA, and I thought SSD would spare me these issues.)
It does sound like a failed SSD drive, or possibly its PCIe connection on the logic board.  There are two things you can attempt before taking it in to Apple though.

1) An SMC reset.  This resets the system management controller, which is responsible for directing power flow through the Mac.  A lot of odd issues, not just disappearing hard drives, can be resolved with an SMC reset.  Apple's KB article HT3964 explains this a bit more in detail.


Shutdown your Mac
Connect your MagSafe adapter to a power source and then to your Mac
On the built in keyboard press left Shift + Control + Option + Power button at the same time.  You might notice the MagSafe's LED light change from green to orange, or vice versa, while doing this
Release all 4 keys/buttons at the same time
Attempt to power on your Mac as normal by pressing the power button


2) Remove the bottom case and reseat the SSD drive.  Obviously this is a bit more involved than performing the SMC reset but it's something I would typically try before taking the time to head to the Apple store.  This does require some specific tools though, a size 5 Pentalobe driver and a Torx 5 driver.  This iFixit walkthrough does a good job showing the process.  Essentially you need to remove the 10 pentalobe screws securing the bottom case, remove the bottom case, disconnect the battery, remove the torx 5 screw securing the SSD, remove and reinsert the SSD ensuring a snug connection, then perform the reverse for reassembly.

Good luck!  Hopefully either of these is all it takes to get you back up and running.